Let's say you have a playbook that should only be executed when the "foo" variable is defined.

---
- hosts: all
  tasks:
    - name: "fail when the 'foo' variable is undefined"
      fail: 
       msg: "the 'foo' variable is undefined"
      when: "foo is undefined"

Invoking this playbook should return something like this. The ansible-doc fail command can be used to show the Ansible documention on the fail module.

When fail evaluates to true, all of the subsequent tasks are skipped.

PLAY [all]

TASK [fail when the 'foo' variable is undefined]
fatal: [server1.example.com]: FAILED => {"changed": false, "msg": "the 'foo' variable is undefined"}

PLAY RECAP
server1.example.com : ok=1  changed=0  unreacable=0  failed=1