Why does my PR say that my branch is out-of-date with the main branch? What does the "Update branch" button do?
"Out-of-date" with the main branch means that the main branch has had some new commits pushed to it since you created your branch
When your pull request (PR) says that your branch is "out-of-date" with the main branch, it means that the main branch has had some new commits pushed to it since you created your branch. In other words, the main branch has advanced forward, while your branch has not yet been updated to include those latest changes. This situation can occur when you work on a feature or bug fix on your branch while others are also making changes to the main branch.

Users with out-of-date branches in their pull requests will see this prompt if branch protections are on and set to Block merge if pull request is outdated.
If you select the Update branch button, it will update your pull request's branch with the latest changes from the base branch. This operation will perform a merge that results in a merge commit in your pull request branch.
Elements of the UI may have changed since the time of posting. For the most up-to-date information, refer to our Documentation.