When you vent, you let something out, whether it's hot air or your feelings. If you vent your feelings, you let out a strong and sometimes angry emotion and just say what you think.
You might vent your rage when your brother once again gets out of doing his chores. You also might vent something to air it out. If it's too hot inside your car, vent it by opening a window. The noun vent refers to a hole that lets air escape. A clothes dryer has a vent that sends hot air outside. A vent in nature is a crack in the earth's surface that lets hot gas and lava escape — like a volcano.