Use this rule to control the indentation.

Parameters

spaces
Defines the indentation width, in spaces. Set either to an integer (e.g. 2 or 4, representing the number of spaces in an indentation level) or to consistent to allow any number, as long as it remains the same within the file.
indent-sequences
Defines whether block sequences should be indented or not (when in a mapping, this indentation is not mandatory -- some people perceive the - as part of the indentation). Possible values: true, false, whatever and consistent. consistent requires either all block sequences to be indented, or none to be. whatever means either indenting or not indenting individual block sequences is OK.
check-multi-line-strings
Defines whether to lint indentation in multi-line strings. Set to true to enable, false to disable.

Examples

With spaces = 1 the following code snippet would PASS:

    history:
     - name: Unix
       date: 1969
     - name: Linux
       date: 1991
    nest:
     recurse:
      - haystack:
         needle

With spaces = 4 the following code snippet would PASS:

    history:
        - name: Unix
          date: 1969
        - name: Linux
          date: 1991
    nest:
        recurse:
            - haystack:
                  needle

the following code snippet would FAIL:

    history:
      - name: Unix
        date: 1969
      - name: Linux
        date: 1991
    nest:
      recurse:
        - haystack:
            needle

With spaces = consistent the following code snippet would PASS:

    history:
       - name: Unix
         date: 1969
       - name: Linux
         date: 1991
    nest:
       recurse:
          - haystack:
               needle

the following code snippet would FAIL:

    some:
      Russian:
          dolls

With spaces = 2 and indent-sequences = false the following code snippet would PASS:

    list:
    - flying
    - spaghetti
    - monster

the following code snippet would FAIL:

    list:
      - flying
      - spaghetti
      - monster

With spaces = 2 and indent-sequences = whatever the following code snippet would PASS:

    list:
    - flying:
      - spaghetti
      - monster
    - not flying:
        - spaghetti
        - sauce

With spaces = 2 and indent-sequences = consistent the following code snippet would PASS:

    - flying:
        - spaghetti
        - monster
    - not flying:
        - spaghetti
        - sauce

the following code snippet would FAIL:

    - flying:
        - spaghetti
        - monster
    - not flying:
      - spaghetti
      - sauce

With spaces = 4 and check-multi-line-strings = true the following code snippet would PASS:

    Blaise Pascal:
        Je vous écris une longue lettre parce que
        je n'ai pas le temps d'en écrire une courte.

the following code snippet would PASS:

    Blaise Pascal: Je vous écris une longue lettre parce que
                   je n'ai pas le temps d'en écrire une courte.

the following code snippet would FAIL:

    Blaise Pascal: Je vous écris une longue lettre parce que
      je n'ai pas le temps d'en écrire une courte.

the following code snippet would FAIL:

    C code:
        void main() {
            printf("foo");
        }

the following code snippet would PASS:

    C code:
        void main() {
        printf("bar");
        }