What does having shortness of breath actually mean?

Physiology: Our muscles use oxygen for metabolism. When exercise exceeds the capacity of the heart to increase its pumping of blood, muscles convert their metabolism to continue working without oxygen but this builds up an "oxygen debt" which must be paid back. We respond to that need by breathing deeper and faster and experience it as shortness of breath. (this ability saved us when chased by a tiger!).

Shortness of breath: We increase our rate and depth of breathing when our body needs more oxygen to be delivered to the tissues. This happens with rest or when anemia or heart failure or low oxygen environment produces a supply deficit. Shortness of breath is the sensation of working to breath in enough air to satisfy the body's demand for oxygen.