I haven't played it since the initial Early Access release, and I am aware that Klei have added massive amounts of content to the game.
You essentially survive on a randomly generated island by collecting food, avoiding the nighttime, and hunting/befriending/hiding from the native inhabitants.
Everything is done with one click of the mouse, collect berries with a click, attack a giant spider with a click, etc. Combat is interesting as they have animation frames, so you dodge their attack, then come in for a quick poke of your own.
Not sure how the free Starve Together content works at this point.