A man walked on the beach, and when looking back he noticed all his crooked steps from wandering all over the beach.
Ten minutes later he looked again, but now he noticed that the sea had "washed away" all his footprints. Completely gone.
In the same way our sins are "washed away" by the blood of Jesus.
I, even I, am he that blotteth out thy transgressions for mine own sake, and will not remember thy sins.
- Isaiah 43:25 (KJV)