A Dirt Bike tank breather vent hose with valve is an apparatus that is connected to a fuel tank, which enables the tank to 'breathe'. It allows for the optimum intake of air as fuel is released from the tank, and expulsion of air as fuel is deposited into the tank (or expands within the tank).
 
Vents are not strictly necessary. But without them you get a vacuum in your tank that pushes against gravity and keeps your gas from flowing freely.
 
Dirt Bike tank breather vent hose with valve for sale online.