I've seen this message in bitbucket a couple times, for some feature branches that branched off from proto.
If I click Sync Now, what does it do?
so
Sync Nowwould merge the all the new commits inprotointo my current feature branch?
It is the "Sync now" feature presented in this 2018 thread (initially for SVN, and for complaining that feature was not visible enough in the new BitBucket Cloud web UI).
In the case of "x commits behind" and Git, it would actually simply move the HEAD of your main branch (master) to the latest of the original repository.
Your feature branch should be unchanged (you can rebase it on top of master after sync if you want).
As mentioned by Ignacio HM in the comments, the BCLOUD-6202 tickets now reports:
Latest update from Atlassian 2024-09-30
Rebasing is now available as an option when syncing feature branches on pull requests, as well as at merge time.
See "Git, your way: Expanded strategies for branch sync & merge"