Ember CLI is the command-line interface for ambitious web application development using the Ember.js framework. It automates common development tasks like scaffolding, building, testing, and serving applications, ensuring consistency and best practices. With Ember CLI, developers can quickly generate components, routes, models, and more, significantly accelerating project setup and streamlining the development workflow. Its plugin ecosystem further extends capabilities, solidifying Ember CLI as a crucial tool for efficient Ember.js development.