Very simple! It's a board filled with leds. You can choose the board size by clicking the up/down button placed in the “Customize the display“ box. The interesting part is that you can control this leds through code! Some players managed to create amazing light shows, you can find their work in the “Popular projects“ section. Release your creativity and you may appear in this section as well! Check the documentation page and let's forge some leds ^_^