Here's a copy paste from an article explaining what a DLC is
DLC stands for “downloadable content.” It’s additional digital content a player can install on top of a complete video game. DLC can be distributed online within the game or via a gaming platform, such as Steam or the Playstation Store. Sometimes, it might be free. Other times, you might have to purchase it separately, or it might be included in bundles that come with the base game.
DLC can encompass a wide variety of things. Some are just simple cosmetic changes, such as skins and voice-overs, while others feature entirely new areas, stories, or game mechanics that completely overhaul the game.
Some content packs add hundreds of hours of additional playing time. Developers often use DLC as a way to update a game and keep players interested for months (or even years) after its initial release.