Totally agree with John.

It means that your errors, sins, mistakes, or crimes will be forgiven and not held against you. A “slate” is a kind of surface that you can write on. The idea in the “clean slate” idiom is that all of a person’s errors, sins, mistakes, or crimes get written down on a slate, but that the slate then gets erased so that it is as if the errors, sins, etc. never happened.