To tell someone where the bear shit in the buckwheat means to explain something to them that they are very likely to know already, especially because it is self-evident.
«I'm not that stupid that I actually need somebody to tell me where the bear shit in the buckwheat: I can see for myself who the bad guy in the film is. Ta!»
A secluded hang out that few know about. where a man and woman can go to fuck and no one will know and there is usually no one around so you can shit in private.