i heard of 'GUI' a lot when scripting and i would like to know what it means
A GUI is a box that allows you to click buttons and open things and open things. Like in driveblox unlimited, you have a GUI that allows you to open and close your house door or sell your house or car
Graphical User Interface
There's an introductory tutorial article on the Offical Roblox Wiki here: http://wiki.roblox.com/index.php?title=Beginner%27s_GUI_Tutorial
Any two-dimensional onscreen button, graphic, or text is a GUI object. For example, the amount of ammo you have left in a gun, your health bar, prompt menus ("Are you sure you want to buy ___?"), etc...
A gui is like Google Chrome or Internet Explorer. It is a box on your screen, that has buttons and text. It isn't 3-D like parts, but it's 2-D. You can interact with it. Like on google chrome you can watch youtube. On your gui, you can use it to open stuff, make things invisible, teleport. It's all up to you. Also, you can add text and pictures.