TL;DR YAML anchors

Two main components: Anchor & which defines a chunk of configuration and Alias * used to refer to that chunk elsewhere.

Simple example, create dwarfs.yaml:

cat<<EOF>dwarfs.yaml
- &dwarf Gimli
- Dain
- Thorin
- *dwarf 
- Balin
- *dwarf 
EOF

When an YAML parser reads dwarfs.yaml the alias will render the value identified by the anchor.

An anchor can be referenced by multiple aliases, and also we can merge maps (to add more values, or override existing ones) i.e.

cat<<EOF>dwarfs.yaml
# anchor
Gloin: &dwarf_basics
  role: Member
  hood_color: white
  skills: &skills_base
    - battle
    - mining

# alias and merge map
Oin:
  <<: *dwarf_basics
  role: Leader
EOF

In dwarfs.yaml file, Gloin is being used as base definition. Oin uses << as key, which indicates that key-values from another mapping should be merged. Important note role value will be override in Oin.