RIP

Why do we say RIP? what do we mean by that? Rest In Peace. Can someone please tell me what that means? When someone dies, where do people, even Christians, think they go? I see tons of Christians say "R.I.P..." We believe they are in Heaven, glorifying God. So, what does resting in peace have to do with Christians? I don't see it. I think Christians should think before we say that...agree?