I baptize you with water for repentance, but one who is more powerful than I is coming after me; I am not worthy to carry his sandals. He will baptize you with the Holy Spirit and fire.John points beyond himself to Jesus. That's what the Church has said about him from the beginning. His mission was to prepare the way, to point people in the right direction. John is typically associated with repentance, turning away from sin, but John himself says this isn't sufficient.
When the Pharisees and Sadducees come to John to be baptized, he tells them to "bear fruit worthy of repentance." It's not enough to avoid sin, as the Pharisees were well known for doing. It's not enough even to rest presumptively on the promises of God, for John says, "Do not presume to say to yourselves..." (As an aside, I think the problem here is that they are holding the promises of God up to the wrong person. The Bible holds in great esteem those who are willing to hold the promises of God up before God who will make good on the promises, but to "say to yourselves" is to talk to the wrong person.) What's needed is an actively good life.
For that we need help. Matthew tells us John is the one crying in the wilderness, "Prepare the way of the Lord, make his paths straight." In turning away from sin, we are clearing the way, making straight the paths -- nothing more. The real action happens when God enters in and together with him we run on the paths of a life lived according to his will.