string is a Jinja2 filter, commonly used to:

• Convert an integer, boolean, or AnsibleVaultEncryptedUnicode object into a string

For example, lets say you use the vars plugin to create a variable named foo that contains an integer.

---
- hosts: locahost
  vars:
    foo: 10
...

The debug module and type_debug filter can be used to verify that the value in the foo variable is an integer, not a string.

- debug:
    msg: "The 'foo' variable contains an {{ foo | type_debug }}"

Something like this should be returned.

TASK [debug]
ok: [localhost] => {
    "msg": "The 'foo' variable contains an integer"
}

The set_fact module and string filter can be used to convert the integer into a string.

- name: convert integer to string
  set_fact: 
    foo: "{{ foo | string }}"

The debug module and type_debug filter can again be used to verify that the value in the foo variable is now a string, not an integer.

- debug:
    msg: "The 'foo' variable object type is {{ foo | type_debug }}"

Something like this should be returned.

TASK [debug]
ok: [localhost] => {
    "msg": "The 'foo' variable object type is unicode"
}