Elite: Dangerous' Horizons season pass gets another string to its bow next week, and yes, for the purposes of this intro, Elite: Dangerous' Horizons season pass is a sort of violin. That 'string' comprises the Engineers add-on, which adds loot and crafting to the enormo-space sim, along with NPC engineers to take your loot and do your crafting for you. You'll be able to improve your standing with these NPCs, in exchange for better stuff. It sounds like a pretty worthwhile add-on, and it's being added to Horizons as a big old patch on May 26.
Even if you don't own the season pass, you'll be getting new content at the same time too—version 1.6 of the base game will get "collectible loot", a new mission system, a mission board, and various other things with the word 'mission' in them.
So that's new missions and whatnot for the base game, and all that plus craftable loot and crafting for Horizons owners. You can read the patch notes for the beta versions of Horizons 2.6 and Elite: Dangerous version 1.6 here.
Horizons' last big bit of content was the Planetary Landings add-on, which added salsa dancing to the space trading/combat/exploration game. That or planetary landings. Probably planetary landings. Five quid/eight dollars has been slashed from the base price of the Horizons season pass, so if you've been waiting to pick it up, now might be a good time.