A part of the body (knee, elbow, head) often represents one of the main functions of that body part. For example, dreaming that you break your leg can represent trouble moving forward or making progress in your life, since the legs are what enable you to literally move forward when you walk. Dreaming that you can't open your mouth to talk might mean you're feeling stifled or somehow unable to express yourself right now.
Occasionally, dreaming about a particular part of the body can result from actual physical pain or other sensation in that part of your body while you're sleeping. For example, dreaming that your head has swelled up 10 times its normal size could be a result of an actual headache while you're sleeping.