Let's say you've a playbook named foo.yml that exists in the /usr/local directory, like this.

/usr/local/foo.yml

The playbook_dir variable will contain the directory that contains the playbook. Here is how you would print the playbook_dir variable.

- name: "output the 'playbook_dir' variable"
  debug:
    msg: "{{ playbook_dir }}"

Running this playbook should produce the following.

TASK [output the 'playbook_dir' variable] 
ok: [server1.example.com] => {
    "msg": "/usr/local"
}