CakePHP is a PHP framework. It's still entirely written in PHP, but it's intended for rapid development of database-driven web applications. Basically, it gives developers a starting point for their code, so rather than starting from scratch every time, they can start with something that has a lot of commonly used code already written (connecting to the database, running queries, sending emails, etc.) and build off of it.
If you're just learning PHP, though, I don't recommend jumping straight into a framework. You really need a solid grasp of the basics before something like CakePHP will be useful to you.
