When looking at the plethora of tiles on your Start screen, the view can get stagnant, despite all the pretty colors.This is where Live Tiles come in. They offer real-time data right on your Start screen, and you don't need to open any apps. For example, the Weather tile will show you the current conditions, and Mail will show you the subject of the latest message you've received.
You can customize which apps are live and which aren't by right-clicking on the tiles. A settings bar on the bottom will pop up with an option to turn the Live Tile on or off. Simply select the preferred option, and you're all set. Note, however, that not all apps have a live, real-time data-streaming option.