Fuel tank cap breather pipe hose.
Your fuel tank needs to let air in to replace the fuel that has been used.
As dirt bikes often end up on their side or even upside down its handy to not have the fuel pour out when this happens.
The CNC fuel tank vent tube solves these problems by having two-way valve that lets air in but stops fuel coming out.
Also the short pipe allows for easy turning of the fuel cap.