Chapter 6. Validate Fonts

Corrupted font files are an old, familiar problem to Mac users. They were corruptible because they were "writeable"—the system opened the files not just for use but also to change ID numbers in an effort to avoid conflicting IDs. That approach, fortunately, fell by the Mac OS X wayside, with font files now strictly read-only. Font corruption reported under Mac OS X is almost solely of old files.

Validation is simply Font Book's way of checking font files for internal problems. It doesn't fix the problems; it merely reports them.