PAR2 files are basically a reverse hash for a given archive. Like if you've ever used sfv files to check the integrity of an archive, they tell you the exact file size for each piece. If the files all have the right size, the files are probably not corrupt.

Taking that one stage further, you can use PAR (and PAR2) files to not only tell if a given archive is correct, but to re-construct the missing bits if it's not.

Like the rules that govern a Sedoku puzzle. Imagine you only have a few number missing from the sedoku grid, as long as you know the rules you can fill in the missing numbers. The more numbers are missing, the more rules you need to work out which numbers go where.

Kinda.