Crossfire is ATI type of double Graphic card, you should see 2x 2.1 Pci-e slots on your mother board, this allows you to have 2 graphic cards instead of 1, and the purpose is that 1 graphic card will run the negative frames and the other will run the positive, this makes the a graphic card carry a smaller burden in running both negative and positive. Once you have 2 you get much smoother quality, and very respectable frame rates! its usually what gaming computers would have depending on the game's graphics really, but don't worry its not a must have xD. Some or majority of the people prefer single I believe... nvm maybe just personal preference of the gaming community.
P.s to crossfire some mother boards or cards require a cable or a connection between the cards ( Search or look it up) The graphic card must also be the the exactly the same as one another
.