When many different people are working on a project simultaneously, pull requests can go stale quickly. A "stale" pull request is one that is no longer up to date with the main line of development, and it needs to be updated before it can be merged into the project. The most common reason why pull requests go stale is due to conflicts: if two pull requests both modify similar lines in the same file, and one pull request gets merged, the unmerged pull request will now have a conflict. Sometimes, a pull request can go stale without conflicts: perhaps changes in a different file in the codebase require corresponding changes in your pull request to conform to the new architecture, or perhaps the branch was created when someone had accidentally merged failing unit tests to the master branch. Regardless of the reason, if your pull request has gone stale, you will need to rebase your branch onto the latest version of the master branch before it can be merged.
