Definition - What does Metro mean?
Techopedia explains Metro
This UI design centers around interactive and colorful tiles with big typography meant to draw attention. It can be used in place of the traditional Windows PC desktop in Windows 8, but is the only UI choice in Windows Phone 7.
The design principle is to consolidate common tasks and processes and speed up their usage. This is accomplished by removing unnecessary graphics and relying on dynamic content to populate the UI. This results in an interface that favors large hubs or tiles over small icons and buttons, with a laterally scrolling canvas to accommodate more tiles.