uuid is a read-only unique identifier assigned by calibre for each book. It is 128-bit unique identifier represented as hexadecimal digits.
uuid would look like this:
Code:
03c78cc3-bdf6-471b-b263-e28ee126965a
Since it is read only, it cannot be updated. The idea behind uuid; is that you can export your metadata from calibre, including the uuid, and then edit the metadata in external editor, after which you can use the uuid in matching the edited metadata to their original book records more reliably than title/author.
Note: uuids was chosen over ids because they are unique across libraries. Using ids might work for people who use one calibre library exclusively. But for people who use multiple libraries, you run the risk of importing into the wrong library and ruining your metadata. This wouldn't happen with uuids, even when you import into the wrong library.