In relation to domain traffic it means "pay per results"
PPC (or Pay-Per-Click) is sponsored advertising where you fix a budget, write an advert and bid against your competition for positions in the sponsored ads area of the search engine results pages. For example, on Google these sponsored areas are down the right hand side and in the blue shaded area at the very top of the page. Once your money runs out your advert disappears.
PPR (Pay-Per-Results) concentrates on organic or natural listings which appear on the left hand side of the page and fill the majority of the screen. Because of the nature of organic listings you are likely to get longer term benefit for your PPR spend as the effects are long term rather than short lived like PPC.