Please note that each indentation level must be precisely 2 spaces. Tabs are forbidden for indentation. Why are indentation even required in Yaml? It is used for semantics or relation. Consider example:
pets: cat: name: furry age: 5 dog: name: barky age: 2 toys: - bone - ball
Here we have two pets, cat and dog. Each of them has name, and age. Dog has list of toys. If we were to use more conventional configuration file format it would be much messier to represent complex relations.
Some other configuration files might represent previous example in following form:
pets.cat.name=furry pets.cat.age=5 pets.dog.name=barky pets.dog.age=2 pets.dog.toys=bone, ball
But consider more complex situation ...
pets: dog: name: barky age: 2 toys: - ball: color: blue size: 70mm - bone: dimensions: length: 10cm height: 2cm taste: chicken
And you have nice mess in your hands ...