Uncharted: The Lost Legacy is not downloadable content, and it's not what was rumoured either. All of the talk pre-release pointed to a standalone story involving Sam Drake, but while it's true that you won't need Uncharted 4: A Thief's End to play this adventure, it's all about the relationship between Chloe Frazer and Nadine Ross instead.
According to Naughty Dog, the new story will build upon the concepts introduced in the main game, including the "wide linear" level design, which deals with much broader play spaces. With the protagonist no longer being Nate, you can also expect a slightly revised feel to the combat mechanics. The story's set after the events of Uncharted 4, and will supposedly be longer than The Last of Us: Left Behind.
All in all, it's looking rather wonderful, isn't it? A release is expected next year.