Bugbug's are annotations that are added to the source code when the developer writing the code isn't sure if the code they're writing is "correct", or if there's some potential issue with the code that the developer feels needs further investigation.
So when looking through source code, you sometimes find things like:
// BUGBUG: I'm sure these GUIDs are defined somewhere but I'm not sure which library contains them, so defining them here. DEFINE_GUID(IID_IFoo, 0x12345678,0x1234,0x1234,0x12,0x12,0x12,0x12,0x12,0x12,0x12,0x12);
The idea behind a BUGBUG annotation is that a BUGBUG is something that you should fix before you ship, but that won't necessarily hold shipping the product for - as in the example above, it's not the end of the world if the definition of IFoo is duplicated in this module, but it IS somewhat sloppy. Typically every component has a P1 bug in the database to remove all the BUGBUG's - either turn them into real bugs, or remove them, or ensure that unit tests exist to verify (or falsify) the bugbug.
As far as I know, the concept of a BUGBUG's was initially created by Alan Whitney, who was my first manager at Microsoft - I know he's the first person who explained their use to me. Lately they've fallen out of favor in favor of more structured constructs, but conceptually, I still like them.