Can somebody explain to me in lay mans term the importance of there fins??

Surfing Equipment & Gear - Surfboard Fins

Surfboard Fins, also known as skegs, act as keels under a Surfboard. These help in steering your board and preventing it from sliding sideways. They also make the board go straight and enable you to turn effectively.

Before, Surfing Boards don’t have fins. Fin setups then include single fin, twin fin, and tri fin or thruster. Nowadays, Surfboard Fins come in different shapes, sizes and colors. Generally, there are two types of fins: Glass on Fins and Removable Fin Systems.

* Glass on Fins
Glass on Fins are laminated to the board itself with fiberglass and are non-removable. Since they are attached to the board, they provide greater strength and better performance. However, they are a bit expensive and are difficult to repair. Non-removable fins can also be a hassle during travel.

* Removable Fin Systems
Removable Fin Systems were developed in the early 90’s but when they first started out, they weren’t interchangeable. Nowadays, this type of fins provides surfers with the ability to enhance their riding skills by changing the size and shape of the fins. One advantage of removable fins is that you can take them out whenever you like. The downside with removable fins is that there is no standard fin system. You have to find the right fins that are suitable for your board.

What about twinser and quad set ups to name but a few more fin styles. Removable fin systems were being used in the late 70s on twin fins. Ive owned some so I know. Glass on fins are actually a lot cheaper than removable fins, thats why boards with glass ons are cheaper to buy.