Shadow of the Tomb Raider Definitive Edition features the base game plus all of the downloadable content (DLC) that has been release post-launch such as the new tombs and extra appearances, weapons, and abilities.
Here’s the official game description and trailer:
In Shadow of the Tomb Raider Definitive Edition experience the final chapter of Lara’s origin as she is forged into the Tomb Raider she is destined to be. Combining the base game, all seven DLC challenge tombs, as well as all downloadable weapons, outfits, and skills, Shadow of the Tomb Raider Definitive Edition is the ultimate way to experience Lara’s defining moment.
Shadow of the Tomb Raider is the last game in the Tomb Raider modern reboot trilogy that explores the origins of the character, Lara Croft. This trilogy began with Tomb Raider and was followed by Rise of the Tomb Raider. Both titles also have Definitive Editions on Xbox One.