An obvious metaphor for a goal is a target. Less obvious metaphors are more interesting, but if you need to draw something quickly during a presentation, obvious metaphors are always at hand.
A target has a bullseye - a point in the very center and rings around it. Usually, the closer the ring is to the bullseye, the better the result. The center of the target is called the bullseye because archers used to practice shooting at bull skulls, and hitting the skull's eye was the most challenging task.
In a black and white target, white and black rings alternate. In a colored one, you can also add red rings.