I am the gate of the sheep. Everyone who came before me was a thief or a robber, and the sheep did not listen to any of them. I am the gate. All who come in through me will be saved. Through me they will come and go and find pasture. A thief comes only to rob, kill, and destroy. I came so that everyone would have life, and have it in its fullest.

Commentary on this metaphor:
This rural metaphor would have been readily understood by shepherds of that day. Once a sheep was inside the sheepfold, it was safe from wild animals or theives. Jesus, like other shepherds, guarded the safety of those who believed in him and protected them forever. This “I am” was meant to assure his followers that he would guarantee their lives from every threat of eternal loss.
Calvin Miller, “The Book of Jesus”