Someone who's plainspoken is very direct and straightforward in the way they communicate. If you ask your plainspoken friend how your new haircut looks, she'll tell you the truth.
Plainspoken people don't use fancy words, go on at length to hear themselves talk, or beat around the bush. They tend to say what needs to be said in a simple, matter-of-fact way. This adjective has been around since the 1670s, sometimes hyphenated (plain-spoken), from plain, "simple or ordinary," and spoken.