Kilns vents are hoods with an exhaust fan that are suspended above an electric kiln. They suck air from around the kiln and expel it through a hose to the outside. Some vents also suck air through a hole in the bottom of a kiln. Vents are intended to remove all harmful kiln fumes from the room.
Kiln vents also have other benefits. They help make for better temperature uniformity at low temperatures giving firings an even start. Below red heat the only way to get heat to move around in the kiln is to move the air (above red heat, the dominant way heat moves is by radiation). They also give better atmospheric uniformity. Continuously bringing in fresh air and expelling products of decomposition makes for more consistency from firing to firing.