Project rules are instructions that explain how work in a particular project should be done. They might say how to run tests, which files should not be changed, or what style the code should follow.
An AI coding tool can read these rules to understand the project's expectations before making a change. They are like a shared note left for anyone helping with the work.
Rules are useful when they are clear and current. If two instructions conflict or a rule is out of date, the person working on the project should ask which one to follow.