It is the difference from the "par" price of the loan that you took. In other words, the broker received a premium from the lender for delivering that particular interest rate on your loan file. For a much lower rate, you would have paid higher closing costs out of your own pocket, and for a much higher rate, you would have paid much less. That is oversimplifying it quite a bit, but I hope that helps.
This is absolutely common and typical in the mortgage industry. Common premiums would be from 0% to 3% of your loan amount.