Network devices can broadcast various services, so other devices can find them. You've probably used this feature before without knowing about it. When you start a Screen Sharing session or browse some files on a network your computer picked up on what other computers in your network were broadcasting.
While its already pretty easy to find these services in Finder, wouldn't it be awesome if you could do this from your menu bar? Or connect to web servers, SSH, printers, see who's in and even create your own services? That's when you need coucou!
Are you a web developer? Check out this post about using coucou to streamline local web development.