Its something they've been developing internally at Basecamp. DHH sees Turbolinks handling "90%" of front end work related to full page changes. Stimulus (which I think he said they're releasing in another month) will handle on page interaction or as he calls it "Javascript sprinkles". Things like modifying existing elements, adding a new node to the DOM, filtering, etc.
I'm far from an authority to officially say, but this is what I gathered from listening to the podcast:
Stimulus essentially structures DOM manipulation. Whereas other frameworks aim to solve rendering, it seems Stimulus will offer a lo-fi and sane way of making your views dynamic. Restated, it's a framework for managing JS sprinkles aka jQuery spaghetti aka templating logic. I don't think its a component library because DHH explicitly mentioned its not a mechanism designed for handling JSON and similar concerns. It targets the realm of updating element classes and attributes, handling events, etc. It'll also allow you to partially update views with HTML templates sent over the wire.
When combined with Turbolinks, it should elegantly solve all your everyday front-end concerns. There's a lot of helpful context I'm neglecting - check out the podcast, DHH pretty much opens with this topic.
Maybe a typo? A search for 'ruby rails stimulus' brought me right back to the root comment. About to listen to the podcast. Will report back if I learn anything.