This is a symbolic representation of the death, burial and resurrection of Jesus.
Subsequent to the New Birth the believer is fully immersed into the water, which symbolises being buried with our Lord. Romans 6:3-7 the believer is baptised into His death on the cross and are no longer slaves to sin. As the believer rises out of the water it is symbolic of the resurrection of our Lord Jesus Christ from the dead. The believer is then raised to his/her new life in Christ.