Tasks are run against managed nodes, also known as hosts.
After a clean install of Ansible, the default hosts file is /etc/ansible/hosts. The default hosts file is completely commented out. If you were to issue command ansible all -m ping, the following would be displayed.
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
Managed nodes are defined in inventory. There are multiple ways to define your inventory: