Something that's upright stands straight up. When you sit upright, your back is straight and vertical. Even your grandmother would approve of your posture.
Most cars have seats that can fold forward or lean back, and then snap upright, back into a normal sitting position; and when you're in class, you're usually expected to sit upright, facing your teacher at the front of the room. Another way to use upright is to mean "upstanding" or "trustworthy," so if your friend finds a wallet full of money and returns it to its owner, you can call her an upright citizen.