in layman terms its when the joints and tendons are stressed, usually due to muscles growing faster than the tendons can which is very easy for a teenager to do. Since tendons are not as vascular as muscles, they do not grow, strengthen, repair at the same rate. Avoiding exercises that cause the 'bad' discomfort is step one. reduction or cessation of trainign volume would be step two. if its in your shoulders I'd cool it on dips, press and bench until the soreness goes away.
I think SL posted up the diesel crew rehab videos a while back, if not a youtube search of diesel crew should get you close. They show the proper way to warm up shoulders and to keep them strong and healthy.