In the Gospel of John, Jesus refers to his disciples as sheep, and to the people who are not yet disciples as lambs. The main difference between feeding sheep and feeding lambs is that sheep can be taught to find their own food, while lambs need to be watched over and tended to. So when Jesus says "feed my sheep", he is saying that his disciples should not just provide for the physical needs of believers, but also teach them how to find spiritual nourishment on their own. And when Jesus says "feed my lambs", he is indicating that there will always be a need for people who are willing to minister to those who are just starting out on their journey of faith.