ANSWER: Baptism is the ceremony that those who are truly repentant of their sins must undergo in order to be forgiven and then receive the gift of God's Holy Spirit. It should also be noted that baptism is a requirement of salvation (Mark 16:16, Titus 3:5).
The full meaning or symbolism of baptism can be understood when one accepts the Biblical teaching that believers should be completely immersed in water (see Acts 2:38, 5:30 - 32, Luke 3:3, 24:46 - 47, Romans 5 - 6 and Colossians 2:12).
Some believe that a valid baptism can be carried out by merely sprinkling or pouring water on a person. This belief is not only unsubstantiated in the Bible, it clouds and obscures what this simple but also profound service truly represents! After a person has come out of the waters of baptism, the Bible commands that a 'laying on of hands" ceremony occurs (see Acts 6:6, 8:15 - 17, James 5:14 - 15, etc.).
The laying on of hands ceremony is usually carried out by those who are mature in age and in the faith (they have been Christians for 10+ years). After baptism, hands are placed on the believers head and shoulders, and prayers are made to God that he (if it is his will) grant the person the gift of his Holy Spirit (Acts 8:14 - 15, 17, 19:4 - 6).
Baptism is therefore a visible declaration that a person has repented of their sins, accepted Jesus as his or her personal Savior, and is willing to commit their entire life in humble obedience and service to God (Acts 2:36 - 38).
The book of Romans, chapter 6, teaches a person's complete immersion in water symbolizes the total death and burial of their 'old self' or pre-conversion way of living. It is through this action that the believer becomes spiritually conjoined into the crucifixion and death of Jesus Christ and has His blood applied as full payment for their sins.
Or are you ignorant that we, as many as were baptized into Christ Jesus, were baptized into His death? Therefore, we were buried with Him through the baptism into the death; so that, just as Christ was raised from the dead . . . we also should walk in newness of life (Romans 6:3 - 4, HBFV).
Rising up out of the baptism waters symbolizes conjoining into the resurrection of Jesus Christ. When a believer comes up out of the "watery grave" after they have been baptized, he or she rises to a newness of life.
Having been buried with Him in baptism, by which you have also been raised with Him through the inner working of God, Who raised Him from the dead (Colossians 2:12, see also verse 13, 3:9 - 10 and 2Corinthians 5:17).