These orings are used on the top of the CO2 tank that threads in to your gun. It's a good idea to put a drop of oil on this oring everytime you screw your tank in. The oring will last a lot longer.
The easiest paintball problem to fix and the most common problem to have. The orings on the top of your tank wear out. They fall off. They get ripped, gouged, dried out, twisted dirty, etc. A bad oring causes leaks that affect your performance.