Plain things are simple, unadorned, and even a little boring. Being a plain talker means you don't use pretentious, fancy words. But having a plain face means you're not much to look at.
Plain comes from the Latin word planum, meaning "level ground." That's one meaning of the word, as in a flat prairie or low lying coastal flood plain. If something is in plain view, it's out in the open with no obstructions. But usually we use plain as an adjective to describe unembellished, bare, or unimpressive things. A white room with just an old bed and a chair is pretty plain.