A soffit vent is a vent installed under the eaves or soffit of your home. It allows fresh air to enter your attic and be drawn out through gable vents, recycling the air and allowing the heat to escape or for cold air to enter and keep the snow/ice from melting causing ice damming.
If your home only has small gable vents or a vents high in the roof, you might want to think adding soffit vents to increase airflow. These vents allow outside air to enter the attic at the lowest point of the roof, along the underside of the eave.
Aside for the obvious energy savings, proper venting will also keep additional aging and damage to the roof from occurring, saving money.
In cold climates, the warm moist air, that escapes from the living space below, can linger and condense on the underside of the roof causing rot. Warmer climates have the problem of severely hot and possibly moist air building up and the need for that air to escape the attic is just as important. Most new homes already have soffit venting included, however older homes can benefit from the addition of soffit vents to increase energy efficiency.