YAML, which stands for "YAML Ain't Markup Language," is a human-readable data serialization language commonly used for configuration files and compatible with any programming language. It is a superset of JSON, offering more flexibility by using indentation and newlines to define structure, making it cleaner and more readable compared to JSON's brackets and braces. At CircleCI, YAML is utilized for specifying configurations, demonstrating its practical application in real-world projects. YAML supports a range of scalar types, and its syntax allows for multi-line strings and collections through indentation, while also enabling nested structures for complex configurations. When creating YAML files, it's crucial to use spaces instead of tabs, ensure consistent indentation, and follow proper syntax to avoid errors. Best practices involve using quotes for special characters and adding comments for clarity. These guidelines help maintain error-free, readable, and easy-to-debug YAML configurations.