Two new passive festivals, a new farm, a new mastery system, new winter outfits, year 2 festival dialogue, a prize machine for quests, a big tree that ends up with a post-community centre questline and some new neighbours, a whole bunch of craftables and machines...
Or you could just look here:
Stardew Valley was released on 26 February 2016 for Windows PC after almost four years of closed development. Linux and macOS compatibility was released in a free update on 29 July 2016.
stardewvalleywiki.com