It's only available for orgs, not for personal repositories, and when we tried to use it in my team the size of our OIDC claim meant something broke so we had to turn it back off again :P. So I didn't get much experience with it.

But it seems like it gives you a place in a queue, meaning you don't need to keep rebasing as earlier PRs get merged: you can see what the state of the main branch should be by the time your PR reaches the head of the queue and develop accordingly.

https://docs.github.com/en/repositories/configuring-branches...

Graphite app helps with stacked PRs and has a good explanation. Not affiliated just a happy user