They were hungry and knew He had to be hungry also, but Jesus gave them an answer that floored them. The disciples had the habit we all have-looking at food as our sustenance. And food does nourish our bodies. But Jesus wanted to make an object lesson here. He was telling them very simply-true nourishment is doing God's will.
True nourishment feeds one's soul, one's spirit.
Physical food only nourishes the body, but the true nourishment rejuvenates one's soul and spirit and makes one stronger spiritually. So you become the slave of that which you obey. But now that you have been set free from sin and have become slaves to God, the benefit you reap leads to holiness, and the result is eternal life. Verse 22 above shows us why we need to be about the Father's business just like Jesus -it leads to holiness, righteousness, and eternal life. If you do, then follow Jesus' example and make God's will your real meat, your real food.