The cartilage across your knee is known as the meniscus. It not only keeps your joints from grinding together, but it also distributes your weight evenly across your knee so you don't develop injuries. If the meniscus gets torn, generally by twisting or turning wrong, its important to give it some time to heal. Your doctor may recommend ice, compression, elevation, physical therapy, walking and possibly a temporary knee brace. It best to take it easy for a few weeks, and do stretching, mild exercises and walking before running again. After the tear has 2-3 weeks to heal and it is feeling better you can try running and playing soccer, just don't over exert yourself. Take good care of your knees so you can continue your current lifestyle of playing soccer for many years to come!