You have torn the meniscus in your knee. There are 2 meniscal cartilages in the knee, one on the inside and the other on the outside. Each is a triangular wedge that helps take weight from the thighbone to the shinbone. They are like shock absorbers. You have torn one of these cartilages. A tear is often the result of a traumatic twisting injury, but can sometimes be due to degeneration that comes with getting older. This can be an indication of early arthritis in the knee.