It's the name of one of the original modules for D&D (dating back to 1979) and incidentally one that I particularly cherish because it's one of the first I've played back in the 80s.
The reason I'm bringing that up is because it's been adapted to the Temple of Elemental Evil:
http://www.co8.org/community/index.php?threads/the-keep-on-the-borderlands-installation-instructions.11135/I'm a bit late to the party as this was done a few years ago back in 2014 but it's never too late.
I wonder how truthful the mod is to the original. It's pretty much the module that shaped my experience of Dungeons & Dragons (and RPGs in general), in fact the only module that had more an impact on me was The Lost City.
The Lost City was about an underground city lost in the desert where various factions would compete for power while some mask wearing priests would enforce the will of their monstrous deity ruling from the bottom of the pyramid over the survivors above.
In comparison The Keep on the Borderlands is a much more mundane setting with basically a frontier setting and a bunch of caves with monsters showing the limits of cohabitation between different types of humanoids.
Good times.