To quote a friend: a pimp post is a post to introduce an interest or idea to other people in hopes that they will like/follow the interest/idea due to a wealth of information.
The meaning of "pimp" in this context is pretty similar. Like the way a pimp "pimps" prostitutes out to potential client, a pimp post "pimps" information out to potentially interested parties (in the case of fandom, these parties are prospective fans). You could say a pimp post is a sort of primer for a particular fandom -- you lay out the basics of that fandom (the kinds of information you'd find on a Wikipedia page) along with common fan conceptions.
I linked some well-written, well-organised pimp posts in my site references. They're all Asian (sorry about that!), but they're excellent examples of how to organise a pimp post. (Keep in mind, the post doesn't have to be anywhere near this detailed. All it needs is a wealth of information gathered into one place to make it easier for prospective fans/interested parties to learn more.)
And a giant list of more pimp posts: