They mean nothing. They're strings wrapped up in small permission objects, but in the end, they're placeholders for numbers; 1, 2, 4, 8, 16, ...
Logically you grant User with id 12347 WRITE and READ permissions for Book with id 28543867, but you're really granting permissions 1 and 2. 2 doesn't automatically imply 1 like you might think, because that only makes sense in our brains - you can't edit if you can't view what you're editing. And an ADMIN permission should imply READ, WRITE, DELETE, etc. but it doesn't, because to Spring Security it's not an admin permission, it's just 16.
These permissions are only the default set, but you can extend it, remove some you don't need, replace some with one with different names, or ignore the whole set and redefine yours from scratch. As long as the changes you make are consistent with the API, and each permission has a unique value that's a power of 2, then you can start granting permission SLEEP_LATE(8), DRIVE_20_OVER_THE_SPEED_LIMIT_AFTER_MIDNIGHT(32), etc.