length is a Jinja filter used to return the number of elements in a variable or list. For example, let's say you've created the following variables and lists using vars and debug.

---
- name: main play
  hosts: localhost
  vars:
    foo: "Hello World"
    emptyfoo: ""
    bar:[ 'Hello', 'World' ]
    emptybar: []
  tasks:
  - debug:
      var: foo | length

  - debug:
      var: emptyfoo | length

  - debug:
      var: bar | length

  - debug:
      var: emptybar | length
...

Something like this should be returned.

TASK [debug]
ok: [server1.example.com] => {
    "foo | length": "11"
}

TASK [debug]
ok: [server1.example.com] => {
    "emptyfoo | length": "0"
}

TASK [debug]
ok: [server1.example.com] => {
    "bar | length": "2"
}

TASK [debug]
ok: [server1.example.com] => {
    "emptybar | length": "0"
}

This could also be used in a when statement.

---
- name: main play
  hosts: localhost
  vars:
    foo: "Hello World"
    emptyfoo: ""
    bar:[ 'Hello', 'World' ]
    emptybar: []
  tasks:
  - debug:
      msg: foo length is greater than zero
    when: foo | length > 0

  - debug:
      msg: emptyfoo length is greater than zero
    when: emptyfoo | length > 0

  - debug:
      msg: bar  length is greater than zero
    when: bar | length > 0

  - debug:
      msg: emptybar length is greater than zero
    when: emptybar | length > 0
...