Autofill means the pool has a water line plumbed to it with a float valve. When water level drops due to evaporation/splash out the autofill system automatically adds some water, maintaining a constant water level. The alternative is to manually put a garden hose in the pool when the water level gets low. Some folks in hot, dry areas report having to add water every few days. This can also be a problem if you're on vacation and the water level drops below the skimmer as the pump will then start sucking in air instead of water.