Component-Based Php Framework

What does it mean by being a component-based PHP framework.

It means that Yii is modular, extensible, flexible.

A more comprehensive answer: you start with a "basic" Web app and you can add user-contributed extensions and your own reusable modules and components, that extend and change how it works.

Yii is very flexible. You can change its behavior by extending its core classes (instead of changing its code directly).