motorcycle Gas Cap Breather Tube Vent Hose 

Your fuel tank needs to let air in to replace the fuel that has been used. That's why your bike has a hose coming out the fuel cap & usually going into the head stock.
This makes removing & replacing the cap a bit of a pain, especially the screw on type.
Also 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 RFX fuel tank vent tube solves these problems by having a one way valve that lets air in but stops fuel coming out. Also the short pipe allows for easy turning of the fuel cap.

Please note when it arrives the tubing will appear to be too small, it will need immersed in boiling water for a couple of minutes to soften it enough before fitting, this way it will form a much tighter seal & be less likely to be lost during riding.