A GUI (Graphical User Interface) is an object that appears on your screen that you can interact with (like the ROBLOX health GUI which displays (shows) your health). You may ask yourself, "Why are these important?". The answer is quite simple. If you're an elite scripter (or have some experience), you could create a game out of GUI's, instead of bricks. Or you could create a game using both, such as a zombie game that tells how many zombies are left and other crucial stuff.