As your Realtor in Chicago we would have a signed contract, it wold be an exclusive buyer representation agreement, there are other types of contracts and some brokers are willing to work without a contract(that's a different story for a different day). The owner of a property signs a contract with their Realtor who offers a split of the commission to your Realtor. If we signed a 2.5% agreement you would only pay my commission if the seller's broker offered less than a 2.5% commission. This is very infrequent but it can happen with a FSBO(for sale by owner) that isn't willing to play ball. Some auctions may not pay a commission to your Realtor. You would have to pay the commission in these cases but you are saving on the purchase price so it evens out.
My contract is 2 pages and is rather plain English. Have your realtor explain their contract to you, they aren't all the same.
TL;DR most of the time you don't have to pay your Realtor a commission, it is paid out of the sale price of the property. Read your contract before you sign it.