WP-CLI is a command line interface for WordPress that allows users to manage WordPress installations and perform tasks like installing plugins, importing/exporting data, searching/replacing, and more. It has advantages for both single site management and large scale management. For large organizations, WP-CLI can be used alongside orchestration tools like Ansible to automate tasks across thousands of sites, like performing critical updates. The presenter discusses how WP-CLI benefits both individual site owners and large organizations by allowing work to be done more efficiently through automation and at scale.