DLC, or Downloadable Content, adds new content to the game, expanding its story, gameplay, or both. For Ghostrunner on the Switch, you can find and install DLC through the Nintendo eShop:
Just as Alex was getting into the zone, he received a notification on his Switch - an update was available for Ghostrunner. Curious, he checked the patch notes and discovered that the update included some exciting new features, including additional levels and game modes. The update was available as an NSP (Nintendo Switch Package) file, which could be easily downloaded from the eShop. ghostrunner switch nsp update dlc