To add to others' answers: it is or was a regional usage in the United States, originally used in southern or perhaps southwestern states. It didn't spread to the Northeast, where I live, until perhaps the 1980s. It is thought to have originated in the 1920s. When I read "between a rock and a hard place," I always imagine it being said with a Southern or Western accent. Like many idioms, it is fairly mysterious why these particular words are used, or why they should mean what they they do. You can do web searches as well as I can, but I don't think I would trust the answers! One suggestion is that it is a reference to the ancient Greek legend of the Symplegades, a pair of giant moving rocks that crushed ships that tried to sail between them... but I don't believe it. There are quite a number of other idioms and phrases meaning the same thing. One of the oldest, translated from Greek, is "to be on the horns of a dilemma." Another is "to be faced with Hobson's choice."

yes. it means you are in a difficult situation where you have to choose between two equally unpleasant courses of action

It means you're in a difficult situation and any decision you make will have a negative result.