Could you explain what this means?

Downloadable Content. It's a common practice in many games now – additional features that aren't necessary to complete the game. For example, the Deluxe Edition of Heroes 6 includes a unique hero and artifact – classic examples of early DLC. In the latest Mortal Kombat, for instance, the DLC includes 3 characters – Rain, Scarlet, and Kenshi; they aren't overpowered or anything, they're just not available to other players. Unique missions and items in Dragon Age and Mass Effect are also DLC. In the upcoming Deus EX 3, a rocket launcher will only be available as DLC. It's become very popular recently.