When standing on a sidehill lie, it is hard to hit the ball straight. This is because when the ball is above your feet, the lie angle of the golf club changes, and for left-handed golfers, the ball will curve to the right. The same goes for when your feet are above the ball, for right-handed golfers the ball will curve to the left.
You can tell how much this side angle will influence how much the ball will curve by thinking of an airplane. If an airplane is tilted to the right slightly, the airplane will fly slightly to the right. The same goes for your ball. If the ball is slightly above your feet, it will slightly curve to the right.
Use this airplane analogy to read the slope and you can adjust your aim to hit a better shot for all your sidehill lies.