What does John 3:27 mean?

John the Baptist—not John, the author of this gospel—was confronted with the fact that Jesus is attracting many followers (John 3:26; 4:1). Probably, this meant He was growing His ministry faster than the Baptist was. Or it might imply people were leaving the Baptist to follow Jesus instead. Rather than becoming jealous, petty, or angry, the Baptist has the right approach. He's glad to see people leave him behind to focus on Christ.

In fact, the Baptist was called so people would follow Jesus (John 2:23, 29–37; Matthew 3:1–3; Malachi 3:1). Seeing his influence wane as people learn about Christ doesn't mean his efforts were in vain; it means they were a success. And, since this was never about him in the first place, the Baptist is happy. Later, he will compare himself to the best man at a wedding, and Jesus to the groom (John 3:29). Seeing the groom's happiness and attention makes the best man ecstatic, not jealous. For the Baptist, it's wonderful to see Jesus gaining the interest He deserves (1 Corinthians 1:12–17).

This is also an important guideline for Christians in general. As John the Baptist says, all that we have is given by the providence of God. No matter what success we have, we owe it to our Creator. We shouldn't be angry or frustrated when others seem more successful (Genesis 4:4–7); God is ultimately the one in control (1 Corinthians 3:1–9).