Quicksand is really just ordinary sand with a lot of water in it that has been agitated (disturbed or stirred up). In dry sand, the friction between the sand grains gives the sand some strength.

When the sand is over saturated, however, a lot of water fills the spaces between the sand grains so that they slide around much more easily, and the sand has very little strength. If you jump up and down on a patch of quicksand, it can't hold you up. Compare this with jumping up and down on dry sand at the beach. The dry sand has no trouble holding you up. Quicksand won't pull you in, but if you continue agitate the sand and water by moving around, you'll keep sinking deeper and deeper. The "quick" of quicksand refers to how rapidly and easily sand grains move around when sand is over saturated. Earthquakes can disturb sand and water to form quicksand. Shaking of the ground can force groundwater into overlying sand, causing it to liquefy.