Hi there!! Firstly, let me say, you are in for a treat!!! You probably already know that!!!
The magnets that DISers are referring to are pictures that you put magnets on and decorate your stateroom door. If you search "Show your door" on this board, you will find pics of many peoples doors. You can get designs on the Creative board here on the DIS, or design your own.
The FE "game" is when you join with a group of DISers and bring/get treats during the cruise dropped into a fabric Fish Extender with pockets. You can see a pic of a FE by searching picture of FE on the boards, too.
We did both on our 1st cruise and will be doing it again next year. Lots of fun!! My kids enjoyed decorating our door, delivering the treats and finding treats in our FE.
Hope this helps!!