DLC stands for downloadable content. It means extra content for a game that is released separately from the original main package.
What DLC can include
DLC may include new story chapters, maps, characters, weapons, skins, soundtracks, vehicles, levels, missions, or large expansions. Some DLC is free, and some is paid.
DLC versus updates
An update usually fixes, balances, or improves the game. DLC usually adds new content. In practice, a release can include both at the same time.
DLC is often connected with season passes, live-service games, and long-term support after launch.