The next day, the crowd realised that Jesus was no longer with them and they were puzzled by his mysterious disappearance. They got into their boats and went to the other side of the Sea of Galilee in search of Jesus. They went to Capernaum and when they found him there, they asked, “Rabbi, when did you get here?” (John 6:25)
Jesus did not address this question, but he told the crowd that they had missed the significance of the miracle of feeding the five thousand.
The crowd, many of whom were probably poor, had had their stomachs filled, and they were probably hoping that Jesus might feed them again. They were looking at the natural, temporal reality of the miracle, but Jesus wanted them to look further and deeper and see the spiritual, eternal reality of the sign.
Jesus said to them, “Do not work for food that spoils, but for food that endures to eternal life which the Son of Man will give to you.” (John 6:27)
Jesus was offering the people a gift, a gift that only he could give. Just as the barley loaves had completely satisfied the hunger of the crowd, Jesus also promised to completely satisfy their deep spiritual need. Bread is regarded as the staff of life; bread sustains life. Jesus was offering life, spiritual and eternal life.
The crowd did not understand what Jesus was offering and asked, “What should we be doing in order to work the works of God?” (John 6::28) Their question was all about what “works” and “deeds” they should perform.
The Jews regarded salvation as an accomplishment, not a gift. They believed that salvation was achieved through their good deeds, virtuous acts, and pious works. Jesus challenged that understanding by saying that the work that God requires of us is to have faith in Jesus, to have faith in the one who God has authorised and sent to save and redeem the world.
It is simply faith in Jesus that brings eternal life (John 6:29). Salvation is the work of God, not the work of any person!
 An explanation of the important title “Son of Man” here.