This is a presta valve (aka French Valve) adapter which can be used inflate your bike tyre using a regular air pump such as those at the petrol station. The two most common valve types these days are Presta valves and Schrader valves. Schrader valves are most commonly found on bikes with wide rims and are the same style you'll find on most cars. They use an internal spring to allow inflation while keeping the valve closed. This style inflates quite easily with any regular air pump. Presta valves are a lot narrower and are commonly found on higher end bikes with narrow rims. They are a little trickier to inflate as they use a manual locknut to open the valve for inflation. Rather than purchase a specialised pump you can inflate a Presta valve using a regular air pump if you have this handy adapter.
Perfect for easy inflation of Presta valve tyre using a Schrader valve pump
Converts French/Presta valve to Schrader/American valve