Basically small pieces of nylon, teflon or other material attached to the back end of a piston to keep it centred in the tube. Buttons take out any slop between the piston and chamber. Keeps the piston traveling straight and smooth. I have also seen folks groove the piston and add a nylon/teflon ring. To me, that is easier than buttons.http://www.gatewaytoairguns.org/GTA/index.php?topic=25835.0http://eairgun.org.uk/buttoning.html