A capless fuel filler makes refueling your car a little bit easier. Once the gas flap is opened there is no need to remove the gas cap before inserting the hose, instead you can insert the hose directly into the filler. A small spring-loaded flap in the filler neck prevents gas fumes from escaping from the tank, and can actually reduce fumes because there is no waiting between the time the nozzle is removed and when the tank is sealed. Ford was one of the first manufacturers to introduce a system, and in their vehicles the system is interlinked with the keyless entry system. When the car's doors are locked, so is the flap in the capless fuel filler.
You can also purchase aftermarket fuel filler caps for cars not equipped with the system from the factory. While these are not as secure as the Ford system, they are no more insecure than the normal gas cap found on most automobiles.