piston rings,
alright there is three of them unless you have a heavy duty truck
but the bottom one is the oil one all of them play some part in the oil lubrications but the bottom one plays the biggest as if you look closely at it there will be spaces in between them for oil to go through. then the top two are just to keep compression in for the piston when it is on the compression stroke. That is pretty much piston rings there is a lot more info on them but that is probably all you are looking for

-garrett