What's Movable Type?
Movable Type is described (accurately in my opinion) as

The world's most powerful and easy to use blog publishing platform.

In extremely non-techincal terms Movable Type is an application that publishes content to a website. The application lives on a web server so it can be accessed from any computer connected to the internet. That means I can write and edit a blog using my MacBook in the comfort of my living room or the PC in the dining room when I visit my grandmother.