This says that baptism is a washing away of sin. We Baptists do not believe this is magical water of some sort. The water is simply a symbol. Baptism symbolizes the washing away of sin, but what is really washing the sin is the blood of Jesus Christ. It was the blood of Christ shed upon the cross for us which took away sin.
Baptism symbolizes our acceptance of that fact by faith.
In baptism we identify with Jesus Christ. We participate in his death as we go under the water, and participate in his resurrection as we come up out of the baptismal waters. Baptism symbolizes our being born again to a living hope through the resurrection of Christ from the dead. When Jesus was baptized, as he came out of the waters the Holy Spirit rested upon him. So the same happens to us.