From the Book of Job, Chapter 31:1 I made a covenant with mine eyes; why then should I think upon a maid? Custody of the eyes means not looking at a good-looking man or woman. It means not staring and letting one's self fall into sins of lust or envy. It means focusing on God and His Love.
Custody of the eyes means not going to movies which will increase lust or ruin your imagination. It means not reading gossip or indulging in porn. Custody of the eyes is a discipline a child can learn.
It means minding one's own business. It means paying attention to goodness.