EKG . The ekg has waves with amplitude and direction. A q wave reflects a lack of electrical force in a certain direction. This lack of force has some correlation when a change in pattern with a loss of muscle as is seen in heart damage like a heart attack.
Old infarct. A q wave means an old infarct (a previous heart attack) that resulted in permanent death of a part of the heart muscle. Depending on the location, extent, and other co-morbidities, it may or may not cause new health problems. The q wave appears because the muscle can no longer transmit electrical signals through the heart. An EKG measures the electrical activity of the heart so that's how it looks.