Seller concessions are any monies given to the buyer, toward closing costs or pre-paid items. For example say you were to offer $100,000 but wanted the seller to pay $3,000 in closing costs, the seller may only want to pay the agents commission on $97,000 because they are not actually getting $100,000 for the property.
As far as your agents commission, if the property was found through the multiple listing service then they were probably offering compensation. If it wasn't then your agent probably submitted a request for compensation to the seller, then he would make sure to get paid. Hope this helps