The Breather allows air to enter the tank, so no vacuum is created as the fuel level drops, which would prevent the flow of fuel to the carburetor.It seals in the gas emissions and fuel at the same time. It is a metal and rubber part, sold individually, rated as "Easy" to install. If damaged, fuel may start leaking from it, and it must be replaced. The part is attached inside of the fuel cap. Refer to your make, model and diagrams to ensure you're choosing the right part.