Forks determines how many managed nodes (e.g. the target systems) a task will be run against simultaneously. For a better understanding of forks, refer to Ansible - Understanding Forks.
By default, the forks directive in ansible.cfg is set to use 5 forks.
forks = 5
Command line option -f or --forks can be used to specify the number of fork, which will take precedence over the forks directive in ansible.cfg. In this example, forks is set to 10, so that each task in the playbook is executed against 10 managed nodes simultaneously.
ansible-playbook example.yml --forks 10