Okie, so commission is basically money you get in the form of a percent of what you sell. So if you get 10% commission and you sell something that is $100, you will earn 10 dollars. So if his commission is 4% (0.04 as a decimal) and he sells $15,000 then he earns (0.04) x ($15,000) = $600 he earns.
I hope this helps :-)
