You can run Ansible in two different modes:
ad-hoc: perform one-off command on a single host or a group of hosts
structured: execute ansible playbook (script) that performs a number of predefined tasks on remote servers in a specific order
ansible (ad-hoc mode) and you will notice that it expects host-pattern (names of servers to which you want to talk to) and additional options like the command that you want to execute:
Usage: ansible host-pattern [options]
ansible-playbook (structured mode) which needs only the name of playbook as everything else can be defnied within the playbook:
Usage: ansible-playbook playbook.yml
We will start with the ad-hoc mode.