Your clothes dryer tumbles your clothes in the presence of hot air. This process causes fine pieces of fabric (lint) to be produced, which travels through an exhaust vent pipe to exit your home. At the end of the vent pipe is a door flap, that automatically opens when air is coming out of your dryer, and then should close tightly when the dryer is off.

As the lint starts to build up, it can prevent the door flap from closing properly, which allows cold or hot air into your home, as well as pests.