Baptism is a Christian rite or sacrament involving pouring or sprinkling water over the head or immersion in water; biblically associated with repentance and confession of sins; variously understood to sacramentally effect or symbolize purification, regeneration, and initiation into a faith community; variously performed on people making professions of faith.
It's not what secures your salvation. Only believing in Jesus will do that.
As an act of proclamation, baptism signifies a person’s testimony to his or her union with Christ, especially through identification with His death, burial, and resurrection.
Baptism displays that, through the power of the indwelling Christ, those being baptized have already died to their old selves and are now alive through Him and to Him.