It means that there is nothing hidden, nothing extra unseen, good or bad.
People say it when someone asks them for something more, either when they are buying something or entering into a relationship.
If we are selling something, we might say "as is", meaning "you buy it as it is now, the way it is now, how you see it."
So, what you see is what you get.
That is, if you know what you are looking at or looking for. "What you see is what you get" doesn't help much if you are blind.