What do the diffrent fins do on a surfbaord?

Question

I have a 9'6 longboard

Answer 1

Assuming you have a 3 fin "thruster" or whatever the design is called, the center fin provides stability and speed.  The outside fins (generally smaller) provide good maneuverability during turns so the rails don't slide out from under you.

Essentially this has become the de-facto standard configuration because it provides the best of the mono-fin (pre 1970s/80s) and the dual fin designs (1980s/90s) into the "ultimate" power and agility board.

People tweak the outer and center fin sizes and angles to fine tune board performance to meet their own styles.