question: skipping: no hosts matched

[Constey]

Describe the bug
I'ts not a bug, but just a question because im unfamiliar with the ansible command. I try to run it on an separate host and getting this message:

root@server:/home/acc/UBUNTU22-CIS$ ansible-playbook site.yml
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does
not match 'all'
[WARNING]: Could not match supplied host pattern, ignoring: server.fqdn.de

PLAY [server.fqdn.de] *********************************************************************************
skipping: no hosts matched

my site.yml looks like:

---

- hosts: server.fqdn.de
  become: true
  roles:

      - role: "{{ playbook_dir }}"

What should I do to run the playbook on an remote host? Thanks in advance :-)