Fallout 4: Automatron is the first of the three currently announced planned expansions for Fallout 4. Automatron adds robotic enemies to the game (which makes sense, given that the story focuses on synths, and the conflict they sometimes inadvertently, and other times consciously, create in human society), as well as the ability to harvest their electronic corpses to create new robotic companions for yourself.
It will also, obviously, add new Achievements to the game- and now we know what those will be, since they are on Steam right now (with the Fallout Wiki also adding descriptions and instructions for each of them). So basically, I guess you can get a jump start on obtaining all of these Achievements by familiarizing yourself with how you get them in the first place on either of those pages.
Fallout 4: Automatron will be the first of three planned DLC packs in the first wave of post launch support for Fallout 4. It will be followed up by Wasteland Workshop in April, and Far Harbor in May. You can currently buy the Season Pass, and get access to all three of these DLC packs, as well as all future unannounced DLC as well, for $30 until March 1.