Rendered fat is just what scanker said, fat that has been slowly cooked off to remove any impurities. When we kept pigs we rendered down the offcut fatty tissues to get lard.
I'm not sure if you can use rendered fat for candles and such like - I thought only tallow could be used for that, but it would probably work, although you'd have to contain the candle in a tin, the fat will melt far too quickly otherwise.
Check out the living history websites for more info, althoughI'm sure Toddy and the others have loads more info on this too.
ODG