DLC = DOWNLOAD-ABLE CONTENT

Its a type of expansion to the game, if you just have the base game, you wont be able to play certain maps, and unlock certain weapons (1 or 2) per class.

You can buy each DLC saperately,

China Rising
Second Assault
Naval Strike

or you could buy premium membership which unlocks all DLC for you and any other future DLC. In total, i think there's 5 DLC (not 100% sure)