What does John 1:29 mean?

This uses the fourth of John's seven names for Jesus: "the Lamb of God."

Lambs were required as a yearly sacrifice for all Jewish families. This was for the celebration of Passover. This holiday looked back to the days of Moses, when lambs' blood covered the doors of Israel, and allowed God's judgment to "pass over" them (Exodus 12:22–23). Isaiah 53:12 predicted that the Messiah would "bear the sins of many." The connection to Passover is the primary reason Jesus is referred to as "the Lamb of God." His crucifixion also symbolizes other Old Testament sacrifices. Specifically, on the Day of Atonement (Leviticus 16), two male goats were presented for a sin offering. Lots were cast to determine which would be killed and which would be presented alive before God prior to being sent into the wilderness. The high priest would "lay both his hands on the head of the live goat, and confess over it all the iniquities of the people of Israel, and all their transgressions, all their sins. And he shall put them on the head of the goat and send it away into the wilderness by the hand of a man who is in readiness" (Leviticus 16:21). Jesus was taken outside the walls of the city to be sacrificed on the cross, to bear our sins (2 Corinthians 5:21).

This verse refers again to the first of seven "witnesses" in the gospel of John, namely John the Baptist. These seven "witnesses" are people who specifically state that Jesus is the Christ, the Messiah, the Son of God. The others include Nathanael (John 1:49), Peter (John 6:69), the blind man (John 9:35–38), Martha (John 11:27), Thomas (John 20:28), and Jesus Himself (John 5:25; John 10:36).

The events mentioned here occur the day after the Baptist's conversation with the Pharisees' messengers (John 1:19). It's likely that many of the same people were there once again.