Why you should roll your Gluteus Medius
The gluteus medius is important for controlling movement of the thigh (abduction and rotation). A weak gluteus medius will allow for your knee to end up in non-optimal positions while running and jumping, thereby placing more stress on your knee.
How to do it
Lie down on your side and place the foam roller under the outside of your hip. Rotate your body slightly, to hit those fibers that are towards the back of your body.
You can do hip abductions to strengthen your gluteus medius. More information on these drills can be found here: knee exercises.