See Type docs.

Null

Same as in JSON.

my_null: null

Boolean

While JSON only supports "true" for input, there are multiple ways in YAML to get a boolean.

my_bool: true
my_bool: True
my_bool: yes
my_bool: on

See Boolean docs

Numbers

my_int: 123
my_float: 123.4

Strings

my_string: abc def
my_string: "abc def"

my_string: |
  abc def

See more info in the Strings page.