Memestreams are places where people can post and look at memes, that aren't part of the normal submitted memes on imgflip. They're usually grouped around certain topics, or interests.
The good news is that they have unlimited submissions. The bad news is that since they don't show up among the normal imgflip submissions, people won't see them unless they visit that stream.
The user ekult was kind enough to put together a list of some of the memestreams on imgflip. It's in the comments of this meme:
imgflip.com/i/l5jxq