When you suffer from runner's knee, you experience pain on the outer side (lateral side) of your knee, usually some centimeters above the joint line where the thigh bone and shin bone meet. On this spot the tendon of your big buttocks muscle (gluteus maximus) rubs against the outside of the knee joint (lateral femoral condyle) when running. This friction between the tendon and the bone can cause an inflammation of the tendon.
The condition typically develops after a short period of running, 10-20 minutes. The pain starts mild enough but rapidly increases to a point where you must stop running. Usually, the pain decreases rather quickly once you stop running and there are hardly any problems in everyday life.
The treatment is almost always non-operative. Physiotherapy including stretching exercises for the lateral structures of the knee typically and promptly cures the condition.