Baptize
To baptize a person is to admit her or him to membership of the Christian religion. There are many further subtleties of meaning, practice and belief that lie behind the simple definition - and that may be disputed between different denominations. What is written here is intended as a simple outline fro those that know nothing of the matter.
The sacrament (ceremony) of baptism (the noun to do with the verb 'to baptize') is in essence a symbolic 'washing away of [original] sin' to make a person fit to present to God and the Christian community. Baptism is most commonly nowadays (in all the mainstream churches) performed on babies soon after their birth. In this case, the celebrant, holding the baby over a font, or ritual water-holder, pours a small amount over the forehead, to indicate the washing away of sin. Other parts of the ceremony may include undertakings made on the child's behalf, by parents and god-parents; prayers; anointing with oil, with or without signing with the Cross; and various candles, symbolic white robes; and so on.
