Definition
Release notes are documents shipped with each software release telling users what was added, changed, fixed and still broken. Common forms include a changelog file in the repository and GitHub Release pages.
How it works
Maintainers organize entries per release, usually labeled with a version number that often follows Semantic Versioning. The Keep a Changelog convention groups entries into Added, Changed, Deprecated, Removed, Fixed and Security sections.
Usage
Users read release notes to decide whether — and how carefully — to upgrade. When troubleshooting, they are also a way to locate which release changed a given behavior.
Related and confusable terms
Semantic Versioning governs the version number itself; release notes explain the contents of a version. Fixes for bugs are among the most common entries.