Note This is still a work-in-progress so you might not find some specifications which you'd expect. More should be on its way soon, though!
The stable branch is for, well, the stable build which means it will only get PRs for bug fixes and necessary patches.
This means anything that fixes known bugs/vulnerabilites to Espresso or anything that makes its code more efficient without needing to be in the next branch.
This branch is for anything that doesn't fit in the stable branch, which includes any refactoring and feature additions.
When a new release is ready and has been tested to be stable, it merges with the stable branch and a new release is made.