Here's an example of all 3 lights at work.
KEY: The right side of her face (your left) is brighter than the other side. So the key is coming from her right.
FILL: The other side of her face is less bright, The result is that her face appears 3-dimensional.
Again, if both sides of her face were the same brightness, that would be flat lighting. Flat lighting is bad (usually). When you watch TV tonight, notice that one side of an actor's face is always brighter than the other.
BACK: The back light is adding glow to her hair and separating her from the background.