"Behold, the Lamb of God, who takes away the sin of the world!" (1:29). What does John mean by "the Lamb of God"?

John's Gospel doesn't explain it here. John says it on two days: "Behold, the Lamb of God, who takes away the sin of the world!" (1:29), and "Behold, the Lamb of God!" (1:36). The second time, two of his disciples hear him and follow Jesus (1:37). The words Lamb of God come nowhere else in the WEB. Two of Favor's Old Testament chapters set this verse beside their own, each labelled a reading. Genesis 22 sets Abraham's "God will provide himself the lamb for a burnt offering" (Genesis 22:8) beside it, as a pair on the Prophecy Map's Lamb strand, and gives the main ways Christians read the link. Leviticus 16 asks whether the goat sent away into the wilderness is a picture of Jesus, and this verse is in one of its views. These notes point to them and add no reading of their own.