To baptize is the ritual of bringing someone into the religion of Christianity. It’s usually when little babies get their heads sprinkled with water in a church.
The word baptize comes from Greek baptizein which means, "to dip in water." That’s what happens when you get baptized — you get sprinkled with or dunked in water by someone official from a church. Christian parents often choose to baptize their babies, but adults can get baptized, too.