A public declaration of what Jesus has already done.
Therefore go and make disciples of all nations, BAPTIZING them in the name of the Father, and of the Son, and of the Holy Spirit.
The Scripture teaches that all who repent and believe in Christ are to be baptized by immersion in water. Baptism is a public profession of faith in Christ. It is symbolic of the death, burial and resurrection of Christ. It is a declaration to the world that we have died to sin and have been raised with Christ to walk in the newness of life. Jesus Himself took part in this ordinance.
Being baptized means you've decided to accept and follow Jesus, and are ready to declare that decision publicly. Water baptism is kind of like a wedding ring: it's an outward symbol of an inward commitment to something bigger than yourself. It shows we identify with Christ's death and resurrection into new life.