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.”
These men were outwardly prepared for the highest teaching but not inwardly. Jesus, nevertheless, gave it to them, but they would still be unable to receive it as the next verse shows.
These men who were fed yesterday are today hungry again. Jesus tells them to seek spiritual food which lasts forever in the satisfying of hunger, instead of physical food, which satisfies only for a short time. They need to get their priorities straight.
“seal” was placed on a document much as we put our signature on it: to guarantee its authenticity. The Father had placed His seal on Jesus to guarantee His authenticity as the Messiah. These Jews would know the teaching of the rabbis that the seal of God was “Truth.” The Hebrew for “truth” was composed of 3 letters: the first, middle, and last letters of the Hebrew alphabet. God had placed His seal of truth on Jesus by His baptism and the signs He was doing.
Note that the everlasting life which they may receive was going to be given to them by the Son of man. They need not do anything to earn it.