YAML, which stands for YAML Ain't Markup Language, is a human-friendly, data serialization format. It's often used for configuration files, but its capabilities extend far beyond that. It's often touted for its readability and its ability to integrate with multiple languages.
Unlike markup languages, YAML has many features that are helpful for tasks other than just generating HTML. It supports complex data structures, is able to self-reference, support for complex expressions, and more.
It’s particularly popular in systems administration, DevOps, and Continuous Integration/Continuous Deployment (CI/CD) processes because it offers a straightforward format that's easy to use and understand.