Supplemental updates are a little weird, but basically they're a stop-gap fix on an existing OS release. Rather than issuing 10.7.6, or 10.8.3, which are likely currently in development. Apple essentially reissues existing updates with fixes.
The OS X v10.8.2 Supplemental Update is recommended for all users running OS X Mountain Lion v10.8.2 and includes the following fixes:
Resolves an issue that may cause certain Japanese characters to appear incorrectly in Mail
Allows Safari to access secure sites when parental controls are enabled
Addresses an issue that may prevent systems with more than 64 GB of RAM from starting up
Resolves an issue that may cause DVD Player to unexpectedly quit
The OS X v10.7.5 Supplemental Update is recommended for all users running OS X Lion v10.7.5 and includes the following fixes:
Resolves an issue that may cause Time Machine backups to take a very long time to complete
Addresses an issue that prevents certain applications signed with a Developer ID from launching
The Supplemental Update is recommended if you installed the Mac OS X Lion v10.7.5 Update (build 11G56).
