The First Contact story pack adds a lot of new content to the Stellaris, dedicated to contact with primitive civilizations. One of the most impressive things is three new origins that will allow players to play for the first time as initially less developed civilizations with a different way of interstellar travel.
In addition, the DLC includes new options for interacting with primitive civilizations: new events will depend on the level of awareness. Due to the new cloaking devices, it is now possible to create fleets and stations invisible to enemies and primitives.
DLC goes with the free update 3.7 Canis Minor which includes a bunch of new things.
The full changelog for First Contact DLC and accompanying update 3.7.