What does John 3:28 mean?

Early in His earthly ministry, Jesus is attracting a greater number of followers than John the Baptist (John 3:26; 4:1). John responded by saying that God is ultimately in control (John 3:27). Here, he reminds everyone that this shouldn't be a problem. The Baptist made it clear that he was not the Promised One (John 1:20). He was simply preaching to prepare a path for the Messiah (John 1:23). In fact, the Baptist went out of his way to say that the One who is coming would be far better, and far more worthy, than he was (John 1:27).

John the Baptist will directly point to Jesus (John 1:29). He specifically says this man is the One who is coming. The Baptist even appears to tell some of his students to leave and follow Jesus (John 1:35–37). There is no valid reason anyone ought to have thought Jesus' success would make the Baptist jealous. If anything, it's proof that what the Baptist preached was true, and that his mission was a success. John even compares himself to a best man at a wedding, happy for the groom's popularity (John 3:29).