Because you guys can't explain any better than a rock can, I'm going to make a brief round-up.
GUI means Graphical User Interface. GUIs are 2D elements that are being held on your screen. Go in a place and press Esc, this is a GUI. The StarterGui (supposed to be StarterGUI) is simply where you hold the GUIs that every player will get each time their character spawns. I hope that was clearer. |