bonus by stacking items with mastery rating

Your mastery is a passive benefit to your character based on what talent specialization you chose. As noted in the tooltip for each paladin mastery here on Wowhead, you can increase the effectiveness of your mastery bonus by stacking items with mastery rating: http://www.wowhead.com/talent#s

Some classes or talent specializations desire mastery rating much more than others, depending on how powerful wow po the bonus is to their role, but the basic idea is that this passive skill is attractive to you because of the talent specialization you chose