and the prize pool is 1,000 FS for 25 winners, I still don't know what is FS?.
mrtornado is right. FS stands for
free spins. If you scroll down on the FAQ page you shared you will find more information about the
slot of the week competition and how to participate in it. Here is the relevant part to your question:
score points and try to get into the top 25 to… well, get free spins to this slot