The premise is that each player is assigned a value by the hosting company, in this case Fanduel. There are three different salary cap levels, 55k (expert), 60k (normal), and 65k (amatuer). You pick a roster of QB/RB/RB/WR/WR/WR/TE/DEF/K while staying under your pre-determined salary cap. You can pick anyone to build your roster.
There are a number of different formats to play. 50/50 is where you enter into a pool of teams. The top half of the entrants win money ($2 entry wins $3.60, $5 entry wins $9, etc). The leagues are a pool of teams and the top 3 payout... similar to standard season long leagues, but these are just one week long. There are also giant tournaments with thousands of teams and the top so many payout. Some of the big ones pay out lots of money.
The one here is a 2 man H2H. $2 each, winner gets $3.60.
It's really a lot of fun. You can do some free ones to get your feet wet. I can change this one to free... or start another free one if you just want to try it out.