Seller's concessions is a stated dollar amount of money that is paid at closing and used on your behalf. This is normally used for closing costs, however, in some cases it's used for repairs or something that needs to be replaced.
This is not a "gift". This is a legal way to allow you to include closing costs in your mortgage loan. The amount has to be within the guidelines for the type of mortgage loan you are applying for. The way it works is this; if you want 3%, then you increase the purchase price on the contract by that amount and then show the dollar equivalent as seller concessions.
This is good for those that may be strapped for cash or those that expect to live in the home for a long time. It is not good for any short term situations.
This should be discussed in detail with your mortgage representative and your agent before deciding. You should fully understand what this means and how it affects your monthly payment and your equity position in the home you are buying.