Are you referring to the DR that they have now?
Thats the Diminishing Returns. Since I noticed lots here are also saying they think you are referring to the DR. For table top games it was first brought in (the ones with dice and figures and tokens and stuff like that) and then in Xbox games such as KOTOR it came in as well. Multiple different versions of DR but for WoW it is basically so that when you use the same silence or CC over and over again it will lose effectiveness for a while until eventually it will not work at all. Without DR then a mage could sit there and sheep you for hours on end if they wanted or a rogue could do the same with saps, or in arenas then rogues truly would have a never ending stun lock.