Playbooks are Ansible's configuration, deployment, and orchestration language. They can describe a policy you want your remote systems to enforce, or a set of steps in a general IT process.
At a basic level, playbooks can be used to manage configurations of and deployments to remote machines. At a more advanced level, they can sequence multi-tier rollouts involving rolling updates, and can delegate actions to other hosts, interacting with monitoring servers and load balancers along the way.
You should look at `Example Playbooks <https://github.com/ansible/ansible-examples>`_ while reading along with the playbook documentation. These illustrate best practices as well as how to put many of the various concepts together.