Meanwhile the disciples were urging him, ‘Rabbi, eat something.’ But he said to them, ‘I have food to eat that you do not know about.’ So the disciples said to one another, ‘Surely no one has brought him something to eat?’ Jesus said to them, ‘My food is to do the will of him who sent me and to complete his work. Do you not say, “Four months more, then comes the harvest”? But I tell you, look around you, and see how the fields are ripe for harvesting.
(Lectionary, New Revised Standard Version)
These five verses from John's Gospel show us an amazing moment in our Lord's life, but without the thirty verses before them it's not so obvious, so you might like to read them (John 4.1-30). Jesus reminds us that we need both physical and spiritual food. As we see he has just met the Samaritan woman at the well, itself unusual for a Jewish man would rarely speak to a woman on her own, and even more unusual she is a Samaritan. No Jew would give the time of the day to a Samaritan for they had built their own Temple and acknowledged only part of the Hebrew scriptures. Worst of all, after Assyria had invaded the northern tribes they had settled outsiders there, so Samaritans could often be of rather mixed heritage. Yet here we see Jesus having an amazing conversation with the woman at the well. He listens to her and answers her questions. He speaks about the 'living water' that God can give, and ends with telling her he is the Messiah.
The disciples are concerned that he should eat some food, but Jesus has been so refreshed by doing God's will and engaging with the woman, that he can't think about eating food. It seems that he has been awakened to the possibility that his message is not just needed by his own people, but by others like Samaritans. In Mark's Gospel we see another extension of this conundrum for Jesus in the story of the healing of the Syro-Phoenician woman's daughter (see this animated version of the story - Mark 7.24-30).
Jesus wants his disciples to know that there is more to life than food. We are not just physical beings as Jesus points out, and there is a great need for people to be fed spiritually. After Pentecost and the coming of the Holy Spirit Peter and Paul would initiate the mission to Gentiles, where the fields were definitely ripe for harvest.
Lord Jesus Christ,
You remind us this day
that we are both physical and spiritual beings,
and just as we need food for our bodies,
so we need to be fed spiritually.
We thank You for all who have taught us:
those who encouraged us to read;
those who nurtured our faith;
and for the work of the Holy Spirit
who feeds us spiritually when we
open ourselves to His power.
Amen.
You might like to look at this subject in a little more depth: