A rubber gasket to help reduce fuel evaporation from the bottom of traditional Zippo lighter inserts and other brands such as Zorro.

Underneath the fuel flap is space to store a spare flint.

Some advise fully replacing the felt pad when installing this gasket. 

I personally recommend keeping the felt pad inside the insert and pushing it in a bit to make space for the gasket (as shown in picture 4). This will further help reduce any potential fuel loss.