A cusp is an anchor where two Bezier curves intersect at different angles, resulting in an elbow shape. Cusps are created by moving one control point independently of the second control point. You can unlock an anchor's control points so you can move them independently using the Unlocked Cusp button (keyboard shortcut CTRL+6).
After creating the shape that you want by moving the control points independently, you can re-lock the control points using the Locked Cusp button 
(keyboard shortcut CTRL+4). Locking control points locks the angular relationship between the points so that when you drag one of the points, the opposite moves also as required to keep the angle between the two points the same. An anchor's control points are locked automatically by converting the anchor to a symmetrical curve or corner.