The b64decode filter is used to decode a string. The b64encode filter is used to encode a string. In this example, base 64 string Zm9v is decoded.

Notice in this example that Zm9v is wrapped in quotes. When wrapped in quotes, Zm9v is interpreted as a string. If Zm9v were not wrapped in quotes, Zm9v would be interpreted as a variable.

- name: "decoded the 'Zm9v' string"
  debug:
    msg: "{{ 'Zm9v' | b64decode }}"

The following should be returned.

TASK [decode the 'Zm9v' string]
ok: [server1.example.com] => {
    "msg": [
        "foo"
    ]
}