Quicksand is a mix of sand or clay and salt water. When water circulates under sand, the sand becomes unstable and unable to support any type of weight while still seeming to be stable. This makes quicksand hard to look out for and dangerous as you can sink in it.
In video games, quicksand can usually be found in adventure games. The best way to escape quicksand is by jumping but some games punish you with instant distant death for walking into quicksand.