It's just spread rebates. Brokers pad the spread enough so that an IB can get a cut for bringing in clients. In this case, the IB is offering some of that cut back to you.
If you are already planning to open an account with a broker, then it can save you a little bit of money to find one of these rebate programs. There are several out there and each one has its own set of brokers for you to chose from.