Their writeup here https://engineering.fb.com/2024/03/21/networking-traffic/threads-has-entered-the-fediverse/.
Basically, it's pretty much 1-way right now. You post something on threads, it will show up after 5 minutes for any Mastodon account that is following the threads account (assuming your on an instance that hasn't blocked threads). Any "Likes" (stars) that are made on that post on the Mastodon/Fedi side are aggregated and ported back to the threads app. Not much else works for right now.