A Designated Market Area is a region, usually centered around a large city, that determines the programming you'll receive. Markets are usually named for the largest city or cities in the market. For example, the New York DMA includes not only New York City itself, but areas where New York channels are viewed the most. This includes areas surrounding the city, as well as parts of New Jersey, Connecticut, and Pennsylvania.
The DMA you are in determines the local and regional content you can receive.