Seller Concessions are typically used in 100% purchase transactions where the buyer is not putting any money down.
This helps the buyer pay for the closing costs associated with the loan process.

Typically on 100% purchase transactions you are limited to a seller concession of 3% of the purchase price if you are obtaining financing from a conforming lender. This may or may not be enough to pay for all of the closing costs involved, depending on where it is that you are buying your home.
There are also many so-called "Non Conforming" or "Subprime" Lenders that will allow a seller concession up to 6% of the purchase price even on 100% purchase transactions.

Seller Concessions cannot be used towards the downpayment.