Baptism is a Christian sacrament. It is the formal entry into the Christian Church.
Baptism is taken from the New Testament, where Jesus begins his ministry by being baptised by John in the River Jordan. In some accounts, the New Testament tells that a dove appeared, and a voice was heard, This is my son, in whom I am well pleased.
Reception into the Christian Church is done by baptism using water. The minister or priest pours water over the (baby's) (person's) head and says, I baptise you in the name of the Father, and of the Son, and of the Holy Spirit.
See also, What_is_the_baptism_of_the_Holy_Spirit