If light hits you, you are exposed to the light. If you stand in front of a heater, you are exposed to heat. If you walk out into the cold, you are exposed to the cold. If you had a heavy exposure to something, you would be exposed to more of it. A heavy exposure to radioactivity would be very bad, of course, since that meants that much of the radiation hit you. The heaviest exposure would mean the most exposure to something, relative to a smaller amount of exposure.