It is a symbol of a new life beginning for a Christian. The old ways are 'drowned' for ever in the water, so that a completely new way of life following Jesus can begin. The first baptisms happened to adults when they made a decision to follow Jesus. In some churches this still happens, and there is a pool which people are completely dipped in. When children are born into a Christian family they are often baptised as babies and the priest pours a little water over the baby's head.
Baptism comes from the Jewish tradition of having a special bathe before important occasions.