The word "believable" can be used to describe a story that's realistic enough for you to accept. When you see or hear it, it makes you feel like it could be real (although you know that it's not real). "Believable" is mostly used to talk about stories like movies, TV shows, and books. The opposite of "believable" is "not believable". You've probably heard the word "unbelievable", but that word means "amazing" or "wonderful", so its different from "not believable". People don't use the word "believable" to talk about stories, explanations, or descriptions of real events. When someone describes a real-life event and you think that it might be true, you say that it "seems plausible":

That seems plausible.

When it seems like it probably isn't true, you say that it's "not plausible":

Yeah, he says that they agreed to pay him five thousand dollars, but that just doesn't seem plausible.