'Magna-Traction' is a very touchy subject. Mainly because some riders swear by it while others don't see what all the big fuss is about. The concept is that a traditional board has 2 points of contact, when the board is on edge, at the tip and the tail. A magna traction board has waves in the side cut which allow the board to have an additional 3,5, or even 7 contact points along the edge. This means that the board is much more stable than a traditional board at the same length.
If you measured the edge of the board, the same sized board would have a longer edge if it had magna traction. Its a wonderful selling point and brings a ton of business to companies like GNU / Lib Tech and Rossignol.
Is it a nice aspect to have? Yes for sure! It helps the rider feel more stable on a shorter board.
Is it absolutely necessary to have? No, of course not. There are plenty of companies out there that do just fine without this fairly new aspect.