@serenesurface-- There are sacs around the knee joint called bursa. These are filled with fluid and sort of act like a cushion around the joint. They absorb shock during activity and reduce irritation or injury to the joint. But if they are used excessively, they can become inflamed. That's what bursitis means. Basically, when the suffix -itis is added to a word, it means inflammation. So bursitis is inflammation of the bursa.
Usually bursitis goes away on its own with rest and cold therapy. But follow your doctor's orders. An anti-inflammatory pain reliever may also be used if the pain is severe but cold therapy and rest is often enough.