“I tell you the truth, you are looking for me, not because you saw miraculous signs but because you ate the loaves and had your fill. Do not work for food that spoils, but for food that endures to eternal life, which the Son of Man will give you. On him God the Father has placed His seal of approval.” (John 6:26-27)

Here Jesus is pleading with those who are following him to not focus on the temporary things of this physical world, but rather, focus on the Supreme Being, and trust that he is God's representative.

What is "the food that endures" ?

The "food that endures" is none other than ones relationship with the Supreme Being. This "endures" - from the Greek word μένω (menō), which means "to remain" or "to survive - because it is not of this temporary physical world. Our relationship with the Supreme Being is transcendental to this temporary physical body and this fleeting physical world. Our relationship with the Supreme Being is eternal.

The things of this world are fleeting because they rust, corrode, whither and degrade. Nothing is permanent here. And at some point our body dies, and we depart our bodies, leaving everything behind. Jesus is requesting that his students and followers focus their attention upon their spiritual lives, because the spiritual dimension does not whither or degrade. The spiritual world is the permanent world. This physical world is a temporary, virtual world.

In other words, this physical world is like a facade or perverted reflection of the spiritual realm. It is like looking at a pond's reflection of a tree on the other side of the pond. Just consider this reflection. Does the reflection look like the tree? Yes, But is the reflection the tree? No. Furthermore, the reflection of the tree on the pond is upside down, with the leaves at the bottom.

This is much like the difference between the physical and the spiritual realm, because all life comes from the spiritual realm - and each of us is from the spiritual realm. But since we are animating these physical bodies, life is only portrayed proximally within the physical world.

One might compare this to a movie set. The movie set is not permanent, nor is it real in that it is just made up to make the movie look like it is real. The movie set might look like a real life scene but it is not. It is a facade of a real life scene.
 
The physical world is acted out by temporary physical bodies - each driven by a spiritual individual. Just as a person sits down inside a car and drives the car away, each of us (spiritual individuals) is driving a temporary physical body. Once the car has reached its destination (or breaks down) the driver steps out of the car and walks away. The physical body provides the same facility. The spiritual individual steps into the body and drives it for 60-90 years, and then it dies, and the spiritual individual must then move on.

The spiritual world is the dimension where God dwells personally, surrounded by innumerable loving servants who share relationships of love and loving service with Him. Out of love they tend to His needs, play with Him and care for Him. They also care and tend to each other in love. The spiritual world is the place where love rules. It is the place we all aspire for. It is a place where there is no hatred. It is a place where there is no envy. A place where there is complete harmony. This is the place many (such as John Lennon) often imagine and hope the physical world could be.

But these imaginings always fail, because the physical world is the place where the envious, selfish beings dwell. The physical world is like the prison of the spiritual world. It is the place where those who cannot function in the spiritual world go, because we have become envious of God, and desire not to love and serve God and others. Rather, we prefer to serve ourselves, and strive to achieve self-satisfaction (which never comes, because we are innately servants, not masters).
 
The only way we could create such a “heaven on earth” that we strive for would be if we were to transform the consciousness of every resident of the earth to the consciousness of those in the spiritual world. This is in fact what Jesus is asking. He is asking us to return home to God, by first changing our consciousness. He is asking us to focus on God and His permanent world, rather than this temporary physical world.

In his statement, Jesus also indicates clearly his God-given authority. He says that God has authorized  him ("placed His seal of approval"), right after he says the "son of Man" will give them "food that endures to eternal life."

And what does "Son of Man" mean? The Greek word that has been translated to “son” is υἱός (huios). υἱός can indicate a relationship of offspring in the context of a physical family, but in this context, it is more appropriately defined, from the Greek lexicon, as "one who depends on another or is his follower." Thus, the correct translation of  υἱός in this context is "servant" and ἄνθρωπος means "humanity," so the correct translation is not "son of man." The correct translation is "Servant of Humanity."

Jesus is explaining that God empowered him to be God’s representative and guide people back home to God. Jesus is serving God and serving humanity, in other words.

Jesus is not claiming to be God here. He states: "On him God the Father has placed His seal of approval.” How much clearer can this be? He is clearly showing that he and God are distinct individuals. In order for someone to place their “seal of approval” on someone else there must be two distinct individuals. For one individual to approve another individual, there must be one person who approves, and another person who is approved.

It is not as if Jesus is the Supreme Being and God needs to create a fake approval process to boost Himself. God does not need anyone's seal of approval. But Jesus is clearly illustrating that he has received such a seal of approval - as he has been empowered by the Supreme Being to deliver God’s message to these people.

And the "oneness" that Jesus once indicated between himself and God is their unity of purpose: Jesus does God's will, so there is a oneness of purpose. God has given Jesus His “seal of approval” because Jesus is carrying out God's will. This means that Jesus' teachings and activities represents the wishes of the Supreme Being. Jesus said this specifically:
"For I have come down from heaven not to do my will but to do the will of Him who sent me." (John 6:38)
and
"My teaching is not my own. It comes from the One who sent me." (John 7:16)
With Jesus' statement above, he is also indicating that he is empowered to deliver people back to God ("give you"). And how does Jesus give them the "food that endures to eternal life"?

By his teachings. If they (and we) listen to and apply Jesus' teachings, they (and we) can receive what he gives, and be saved.

Being saved is not an automatic process by which we simply say "I surrender to Jesus" and then we are saved. It is not as if Jesus' "dying" on the cross automatically saves anyone. We must apply Jesus' teachings. It is following Jesus' teachings that saves us. And what was his primary teaching?
“ ‘Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment.” (Matt. 22:37-38)