It forces the Lua environment to run a garbage collection. If you don't know what that means, you probably don't need to. It will likely have very little impact for you, as WoW's default garbage collection algorithm runs it fairly often now.
Im curious what this is supposed to do. Ive clicked it and did a force garbage collection but it changes nothing. So whats the purpose of this part of the addon :D
It should probably come with a note saying "For Developers/Testers Only". Regular users have no need to force garbage collection ever. It is more likely to have a negative effect on your performance than a positive. That's because some addons use weak tables to cache data. When you force a garbage collection, you will clean up those caches potentially requiring them to be recomputed at a later, more "inconvenient" time (such as during heavy combat).