See wp:YAML
I learned it when it was first introduced in 2001 so at that time it was known as "Yet Another Markup Language", however the acronym later became a recursive backcronym standing for "YAML Ain't Markup Language". So, what is it then? Well, it's a data serialization format. It is a superset of JSON, which means that any valid JSON is valid YAML.
YAML is used everywhere, such as the Ansible project. If you want to use Ansible, learning YAML first will help you tremendously. If you're ready to jump in, see Working with Playbooks in the Ansible docs.
Resources
10 Things you might not know about YAML
https://www.xkyle.com/A-Detailed-Comparison-of-YAML-Formatters/
Tools
yamllint https://github.com/adrienverge/yamllint GPL3
For more on Syntax rules and examples; plus linting your YAML, see yamllint
Snippets
# This file IS valid YAML because the Jinja is in a YAML comment
# {% if extra_info %}
key1: value1
# {% endif %}
key2: value2