5 But he was wounded for our transgressions; he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his stripes we are healed.
6 All we like sheep have gone astray; we have turned-every one-to his own way; and the LORD has laid on him the iniquity of us all.
Isaiah 53 records the Holy Spirit speaking through Isaiah telling of Christ's death centuries before it happened. We have all rebelled against God, seeking our will rather than his. This is sin. The penalty for sin is death, separation from God for eternity. (Romans 6:23)
Jesus bore that sin. It was laid upon him as he died on the cross. He was sinless. He died for our sins, as Isaiah shows.
That is what we observe on Good Friday.