Tasks are run against target servers. Some Ansible documentation refers to the target servers as "hosts".
After a clean install of Ansible, the "inventory" directive in ansible.cfg is commented out, like this.
#inventory = /path/to/hosts
In this scenario, the default hosts file is /etc/ansible/hosts and the default hosts file is completely commented out. If you were to issue command ansible all -m ping, the following would be displayed. Likewise, if you were to uncomment the "inventory" directive in ansible.cfg without defining your inventory, the following would be displayed.
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
Typically, target servers are defined in the default hosts file or your own inventory file. Sometimes, the "inventory" directive in ansible.cfg is uncommented and updated to point to the directory where the default hosts file or your own inventory file will be located.
Additionally, Ansible uses inventory plugins to parse inventory. For example, ansible.cfg may have the following.
enable_plugins = host_list, yaml, ini
For example, the yaml inventory_plugin allows you to define target servers in a YAML default hosts file (/etc/ansible/hosts) or your own inventory file. For example, let's say /etc/ansible/hosts contains something like this.
Here is how you can run the example.yml playbook using the target systems specified in /etc/ansible/hosts.