The ansible-pull command is a special feature of Ansible that allows you to, all in one go, pull a playbook from a Git repository (for example, GitHub) and then execute it, hence saving the usual steps such as cloning (or updating the working copy of) the repository, then executing the playbook. The great thing about ansible-pull is that it allows you to centrally store and version control your playbooks and then execute them with a single command, hence enabling them to be executed using the cron scheduler without the need to even install the Ansible playbooks on a given box.
An important thing to note, however, is that, while the ansible and ansible-playbook commands can both operate over an entire inventory and run the playbooks against one or more remote hosts, the ansible-pull command is only intended to run the playbooks it obtains from your source control...