The gluteus medius (nick name glute med) is one of the three gluteus muscles. It is the second largest of the three. The glute med abducts the femur bone, meaning it raises the leg out to the side, away from the center line of the body. It also stabilizes the pelvis during walking and running by keeping the pelvis relatively over the hip joint and thigh bone. If the pelvis sways way out to the side when taking a step, chances are high your glute med is weak.
See: Strong Hips = Strong Knees to learn a strengthening exercise for the glute med.