For the Jewish People, the sacrifice of an animal in the Temple (often times a lamb) was an important ritual. It expressed their desire for the forgiveness of their sins. Prior to entering the Temple, they would perform a ritual bath called "mikveh" as a sign of their desire to be clean before God.
John the Baptist was the son of an Old Testament priest who were the ones that offered sacrifices in the Temple on behalf of the people. So he of course knew well of the "mikveh." When he baptized people in the Jordan river, he did so as a cleansing for the repentance of their sins. But he did so in order to prepare the people to recognize Jesus as the New Lamb of Sacrifice, the one who would offer himself to God so that we might know our sins were truly forgiven.
So when he saw Jesus, he proclaimed, "Behold! The Lamb of God who takes away the sins of the world!" But what does this mean for our life, for our faith today? The knowledge that we are forgiven of our sins is so foundational to our relationship with God. Without knowing of God's mercy, how would we be able to have a friendship with God? We must come before the Lord knowing in our heart that our sins have been forgiven. So now we too can exclaim with John the Baptist, "Behold the Lamb of God who takes away the sins of the world."