"Content of the season." Meaning DLC's for one year I think. the "Horizons Season Pass" also gives you the Horizons DLC, which allows planetary surface exploration, and the associated missions - the stuff with the SRV.
It does not get you cosmetic upgrades and suchlike found on the store, it does not get you beta access to any of the 2.X updates and it does not get you access to any season 3 updates onwards although core changes to things like missions and stuff will filter downwards to everyone.