[throw4321]

People breathe, so you need outdoor air to replenish the oxygen and get rid of the carbon dioxide. That's "fresh" air.

Unfortunately, outdoor air has particulate and ozone pollution. Filtering it gives you "clean" air.

In winter and summer, you also heat or cool the indoor air for comfort. If you just pump in outside air, you effectively also pump out the indoor air. This wastes the energy that had gone into heating or cooling it.

These systems save that energy by transfering heat between the air that's getting pumped in and the air that's getting pumped out.