A good question for someone not familiar with real estate terms. Briefly, it is an agreement between buyer and seller to give a credit toward something regarding a house.
It benefits a seller in that it MAY help sell the house. Example-- you have your downpayment money but lack closing money. IF the house will appraise high enough the seller consents to include the amount needed (usually up to 6% of selling price) in the selling price thus giving you the needed funds by "paying " them for you. You still wind up paying for them indirectly in the price of the house.
Aside from closing costs a seller can give you a "concession" to replace/repair things that they can otherwise deal with if the house does not sell. Example--carpeting/flooring, a septic upgrade, appliances. It is an incentive to get you to buy it knowing you will get funds to do it on your own.