First a little muscular anatomy terminology would be useful here. A flexor muscle is a muscle that decreases the angle between two bones that meet at a joint via contraction. An extensor muscle is a muscle that increases the same angle at the joint. A crossed extensor reflex is an autonomic withdrawal from a danger stimulus. For example, if a person stepped on a sharp object with their bare left foot, the body would undergo a crossed extensor reflex to withdraw the left foot from the object and put the body's weight back on the right foot. So the flexors in the retreating left leg contract, while the extensors in the right leg contract, thus shifting the body's weight accordingly to remove the danger.