sort is a Jinja2 filter, used to sort an array (also known as a list). Let's say you are using the vars plugin to create an array of integers, like this.

vars:
  integers: 
    - 20
    - 3
    - 1

Or the set_facts module, like this.

- set_fact:
    integers:
      - 20
      - 3
      - 1

The debug module can be used to output the entire array, like this.

- name: output the contents of the 'integers' array
  debug: 
    msg: "{{ integers }}"

Which should return the following. Notice the output is not sorted numerically.

TASK [output the contents of the 'integers' array]
ok: [server1.example.com] => {
    "msg": [
        "20",
        "3",
        "1"
    ]
}

The sort filter can be used to sort the list, like this.

- name: output the contents of the 'integers' array, sorted
  debug: 
    msg: "{{ integers | sort }}"

Which should return the following. Now the output is now sorted numerically.

TASK [output the contents of the 'integers' array, sorted]
ok: [server1.example.com] => {
    "msg": [
        "1",
        "3",
        "20"
    ]
}