The Meniscii are two C shaped structures located on top of the tibia that are important for knee function. It is often torn during sports or as a result of age/wear and tear etc. The outer part can heal but the inner half has poor blood supply and does not heal. Many tears result in pain, clicking, locking, swelling and giving way. It is not well seen on x-rays and best seen on MRI scans.
If this is injury is disabling it requires Arthroscopy (keyhole) surgery to be sorted out.