Baptism set him apart from other prophets in the past, and other reformers in his own day. But even in terms of baptism, John says, the Messiah exceeds him. It's important to understand this basic meaning, regardless of our own church's practice of baptism, or how much water our church may use. It is a baptism of repentance still, since by submitting to Christian baptism, a person either tacitly or explicitly acknowledges his need for cleansing, and therefore his sins that demand forgiveness and cleansing. John immerses his disciples in water, while the Messiah will immerse his followers in the Holy Spirit.
Church doctrines aside, the basic meaning is to immerse or dip a person in the Holy Spirit.
To flood a person with the Holy Spirit. Water is just a symbol of cleansing. The Holy Spirit, on the other hand, is the actual agent of cleansing and empowerment. Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. This may be similar to the analogy of baptism, the idea of covering completely with.