One of the seraphim takes a burning coal from the altar. This, of course, strengthens the idea that there is an undescribed sacrifice on the altar, and that the burning of that sacrifice is producing the smoke. When the seraph takes that coal and touches it to Isaiah’s lips, he notes “thine iniquity is taken away, and thy sin purged.”
This image has two functions. The first is to explicitly cleanse Isaiah so that he is among the pure that are capable of being in the glory of God. It is also a reference to the sacrifice, and the function of the sacrifice. At the same time as Isaiah is purified, he strengthens the connection between sacrifice and the atonement for sin. Thus God, through the sacrifice on the altar, has cleansed Isaiah of his iniquities.