What is it?
A stitch is a sharp pain felt in your side just under your ribs. It is commonly felt while running. It can be painful and cause you to stop exercising. Although it is not clear what is the cause of a stitch, it is thought that these pains in the side are a result of inadequate oxygen delivery to the muscles that are involved in breathing - the diaphragm and the intercostal muscles (between the ribs).
Some people try and 'run trough' the pain, hoping it will go away after a time which it sometimes does. Breathing through pursed lips is recommended to ease the pain, but stopping your exercise activity is sometimes the only solution. The pain usually recedes after a time, and there should be no permanent damage done.