FCS (Fin Control System)
is a brand, as well as type, of fin set-up which allows for more versatility in the placement and style of the fins attached to a surfboard. The FCS style can be used as the traditional tri-fin placement, a four (quad), or a five fin set up. The tri-fin refers to a setup with three fins on the underside of the board, often with the two outer fins angled inward and middle fin a few inches closer to the end of the tail pointed straight and larger. The five fin placement is similar but with four outer fins and one middle fin (these fins are often smaller). The quad fin setup has 4 outer fins but no middle fin.
Some styles enable the rider to adjust the fore and aft positioning of the fin. The injection molded fins are designed to break off in the event of collision to minimize board damage and injury to the surfer
More about FCS HERE