The quads are a group of 4 muscles found in the front of the thigh. Their main function is to straighten the knee joint from a bent position. The rectus femoris muscle, the biggest of your 4 quads, also works to flex the hip so that you can lift your thigh up, as when you walk up the stairs.
To engage your quads, try a wall squat. Stand with your back against the wall, then slowly bend your knees as if you are sitting in a chair. Hold for 30 to 60 seconds and release.
-Diaz, Cameron. The Body Book: The Law of Hunger, the Science of Strength, and Other Ways to Love Your Amazing Body. N.p.: HarperCollins, n.d. Print.