Let's say you have a playbook that has a task that you want to ensure does not produce any output and is not logged. For example, let's say you are using the set_fact module to define the "secret" variable.

---
- hosts: localhost
  tasks:
  - set_fact: 
      secret: "itsasecret"
...

By default, the value "itsasecret" will not be included in the output.

~]# ansible-playbook example.yml
PLAY [localhost]

TASK [Gathering Facts]
ok: [localhost]

TASK [set_fact]
ok: [localhost]

PLAY RECAP
localhost  :  ok=2  changed=0  unreachable=0  failed=0  skipped=0  resuced=0  ignored=0

However, let's say the ansible-playbook command is run with the -v (verbose) flag. In this scenario, the output will include "itsasecret".

~]# ansible-playbook example.yml -v
TASK [set_fact]
ok: [localhost] => {"ansible_facts": {"secret": "itsasecret"}, "changed": false}

The no_log:true parameter can be used to supress the output.

---
- hosts: localhost
  tasks:
  - set_fact: 
      secret: "itsasecret"
    no_log: true
...

Now when the playbook is run, even with the -v (verbose) flag, "itsasecret" will not be included in the output.

~]# ansible-playbook example.yml -v
TASK [set_fact]
ok: [localhost] => {"censored": "the output has been hidden due to the fact that 'no_log: true' was specified for this result", "changed": false}