Normally, your computer controls how your Christmas lights are animated. You create animations with the xLights software, then it sends commands to your light controllers and strings of strings to animate the them.
While the convenience of having a single computer for creating animations and animating your lights is nice, leaving your computer powered on all day during the Christmas season is not that efficient. If you accidentally turn off your computer, reboot it, the computer crashes, etc., your light show stops. Powering a computer 24x7 is not efficient with electric, either.
Some people solve this problem by creating your animations using your home computer, then playing back the animations and controling your lights using a separate, low-power computer. This way you can continue to use your home computer as usual throughout the day, while the low-power computer is running 24x7, dedicated to playing your light show. The Raspberry Pi is perfect for this purpose. In xLights, you save your animations as playback files, then upload them into a special program (Falcon Player) on the Raspberry Pi that simply plays the files.
The Falcon Player software is quite powerful. With a little bit of tweaking, not only can it play animations created from xLights, but it can allow Internet control of your light show! By saving the xLights animations as multiple playback files (one animation per file, for example), people visiting a webpage can click buttons that tells Falcon Player which animation file to play. For instance, an animation can make your lights look like candy canes with red & white lights, while another animation file can make your house sparkle with twinkling lights. When no one is actively "controlling" your lights, a "regular" animation file is played for cars passing by.
The nice thing about Internet control of your Christmas lights is that is uses your existing knowledge of xLights (for creating animations). And if you are already using Falcon Player to playback the animations, you don't have to learn much new there, either! Internet control is mostly about tweaking Falcon Player to accept commands from the Internet and it's not that hard to do!