Crowds had gone to wilderness to be baptized by John. They were expecting a messiah and many thought John was the one. John cleared up that misconception for them. It occurs to me that we moderns still may be confused about the source of our salvation--financial security, a fence with a locked gate, a more youthful visage, etc.
John told them that the Messiah was going to make judgments.
It might be instructive for us to reread verses 7-14. The crowds fearful of the coming judgment have asked John what they should do to escape the wrath to come. John tells them to share, to be fair, to be honest, and not to be greedy.
As I read verse 17 and picture the Messiah coming with a winnowing fork, I fear the possibility of being one of the chaff that gets blown into the unquenchable fire. What I hope John means is that the Messiah can blow away those parts of me that are not generous or fair so that what is left is a person whose baptism has made a difference to her and to the world she lives in.