It's literally the pleb version of Dwarf Fortress. It sacrifices some mechanical and detail depth, but compensates with an actually easy to use UI and a workable combat system, as well as with actual graphics. This makes the game accessible and actually allows you to care about your pops. Sure, DF might have a detailed description of appearance, personality and life of each dwarf, but you're not really going to bother with all of that, are you? Meanwhile RimWorld pops are far more relatable, both because they aren't just ASCII sprites, but also due to them being more rare to find, and more individually useful.
The main goal of the game is to build yourself a spaceship and escape from ancapistan, while dealing with all the crazy shit that happens in between. Also it is fun to RP as a certain society IMO. Here's a rundown of the more interesting things from my last game:
>To get mats from trade I decided to start a drug lab, producing stims, joints and crack>Lab was cut into the mountainside, so it would constantly get infested by space termites>Can't give bug meat to people, so instead I cut it to kibble and feed it to my livestock>Open an ancient toomb in my area. Filled with cryopods. These contain a bunch of ancient super soldiers. Knock them all out while they got cryosickness, rob them of their tech and send them away. Now I'm armed to the teeth. >One pop can't handle pain. Constantly collapses from the smallest problem, like squirel scratches. Prescribe them a diet of weed, which kills pain, and allows them to function in case of small injury. >Other pop goes a bit crazy, starts binging drugs. Instead of the safe options like weed or spirit tea, they pick crack. Order another pop to beat the shit out of them before they OD. >Buy a gang of worgs from a trader. Don't have an animal trainer, so after a year they go wild and attack me. >A bunch of other fun stuff.