This is a set dollar or percentage amount put towards the price of the home that the seller agrees to contribute to you, in order for you to put towards the closing costs of the home.
For instance, both parties have agreed on a purchase price of $200,000. You then ask the seller for a 3% seller concession. They will then add this 3% (equivalent to $6,000) to the price of the home, equaling $206,000. The seller will then give you that $6,000 when the sale takes place, so that you can use it to cover the closing costs. Note that you repay the seller concession over the life of the loan.