A gable vent is an opening that allows air into attic spaces of a home. They may have fans attached, or they may be decorative.
A gable is where the roof of a house comes to a peak. The vent is usually on the end of a house or over a garage, and some houses have several gables and several gable vents. They are usually made out of wood with screen wire backings to keep bats and other pests out of the house. They come in several sizes and can be trimmed to fit various spaces, so they are very useful in houses with high-pitched gable roofs.
Attic spaces trap air and moisture, which causes mildew and mold to form. Air trapped in attics is often extremely hot and can be a fire hazard. Gable vents allow this air to escape to prevent fires and moisture. If wind is working with the vents, then the vent is effective. When there is an absence of wind or it is blowing in the opposite direction, the vents are not as effective. In 2015, building codes and standards are stricter than in the past, so some older homes may not have gable vents.