Sure, ask away! Let see...
A commission is when a person ask the artist to create something just for him/her. Like with me, I dont have any pre-finished spiders to sell that I can just ship of when anyone want them. Instead when a person want a spider thats then I create it. You could say that you hire a artist to custom make something especially for you. Thats a commission.
I hope I explained it understandable.