A checksum is basically a number calculated from the data in the file - any changes to the file will alomst likely produce a different checksum, so if when comparing the calculated value for the current copy of the file against the known good internal checksum they're different, then you know the file has either been tampered with or is corrupt.

In the case of FF5 though, the bad checksum isn't because it's corrupt (or shouldn't be ), it's because the ROM has been tampered with Basically because it's a translated ROM, and not an original, and the checksum itself wasn't corrected after patching the ROM with English dialog Unless you really want to play it in Japanese, then it's not really a problem in this case If you got a bad checksum for the english FF6 ROM however, and you know it should be an original and not modified, then it might be a bad sign