Commission is a charge that a hotel pays to third parties (travel agents, OTAs) for selling their hotel rooms. It is usually a fix percentage of the room rate.
The most common commission is 15%. It is also the average commission Bookings.com charges the hotels. In certain locations where the market is stronger, they have introduced a 'preferred property' scheme with a higher % for more exposure / a spot in the top 10 properties for that location.
Booking.com also works with a performance model, so that you pay higher comissions as a hotelier more visibility you get.