Curmudgeonly barbell expert Mark Rippetoe is not given to subtlety—which is a very good thing when you’re learning how to squat.
Coach Rip defines good form in the squat as follows: the ability to keep a barbell exactly over the middle of the foot throughout the entire range of motion. Doing so eliminates the “lever arm”—or “moment arm”—between the bar and the middle of your foot. If the bar deviates from this position, you create a lever you have to work against in order to balance the load.
Other moment arms, such as the distance between your hips and the bar, represent the resistance you have to overcome during the movement. The key to the exercise: overcoming the inherent moment arms without introducing new ones created with bad form.
Additional reading: Low-Bar vs. High-Bar Back Squat by Mark Rippetoe, published May 1, 2008.