Check your setting ('serial'). If you are including the setting under your 'hosts' section ('serial: 1'), that will cause Ansible to execute the commands sequentially. Otherwise, Ansible will execute the command in parallel across all hosts (or at least as many as the available resources will allow) in your inventory. Keep in mind that it will do that only a command at a time, meaning that a single play/tasks execute that way, if one host finishes before the others, it will wait until ALL hosts are finished with that task before any host will receive the next task. Keeps things running in a consistent order in case there are dependencies.