This verifies the integrity of certain game files to ensure there's been no loss of/alteration of data in those specific files.
Good for error checking, if a download may have failed, or if your fancy custom Game.exe has been changed.
As noted in the script, this is not a dependable way of preventing modifications. It's pretty much the easiest buffer to stop people who don't know what they're doing from changing stuff up but like with every security measure, it can be broken if they bother trying.