Best would be to read the comprehensive documentation being provided on the Ansible website.
If by "effect" you mean whether a change will be made after running a playbook, I'd suggest using the 'dry-run' (-C, --check) option.
To understand what plays and tasks are part of a playbook, I might use the --list-tasks option and look closely at ones that stand out to me... reading the ansible-doc information for any modules I'm unfamiliar with.
The best option to monitor the execution of the playbook is to use --step. This would stop at every task at show the result of the execution.
Possibly; however, I take the question to mean the user want to figure out what should happen as opposed to stepping through the playbook.