Generally, a game — even a procedurally-generated game — begins with an idea of what you can do and how exactly it expects things to play out. AI Dungeon is not one of those games. It's a text-based adventure game where you can type in any command you want, and the game will find a way to just roll with it.
The game starts out by giving you a choice of setting and character — things it'll base the beginning of your story on, but kind of forget later if you let things go too far off rails. Since it's a game that lets you type any command and it'll try to find a way to move forward, going off-rails is kind of to be expected.
The game is the brainchild of Braydon Batungbacal, a developer and entrepreneur who has a bit of an eclectic history. He briefly worked on a "Poke Radar" app for Pokemon go, and also co-founded a Seattle-based vehicle-sharing service that lets people rent electric-powered mopeds that he hopes will one day be self-driving.
It seems that, for all Batungbacal's projects, AI Dungeon is the first game that he's has ever released, and it only loosely fits that definition. The situations it presents you with don't always have an immediately-obvious goal, and there's no real endpoint or way of measuring how well you've done. The main draw — and it's quite a draw — is the ability to futz around in an AI-driven world and discover all the weird things that happen.
That may not be enough to entertain people who go into a game looking for quests to complete or any overarching sense of purpose. In my first game, for instance, bandits kidnapped my sister, so I spent time searching for her — only to finally discover that she was... at school. Frustrated, I decided to type "blow up the world," only for the game to tell me that I did, and then have someone say "good job."
The game is available on the App Store and Google Play, though for some reason it currently doesn't always show up when you search for it. Searching Braydon Batungbacal's name in the stores will bring you better results.