explain of gui confused??

G.U.I. stands for Graphical User Interface. A graphical user interface that can contain information on parts or on the screen of someone's client. In Roblox, people commonly put their ScreenGUIs in StarterGui in order to make a menu, rules, or game passes on a game when they join the game or reset their character. There are many different types of GUIs that you can use for different use cases.

You can use a SurfaceGui to put a GUI on a part for labeling a button, making a leader board, or any use case that you want the information to be physically in the game and to adapt with the player's point of view. If you want it to be on the clients screen, you can use a ScreenGui to make it happen as stated above. If you have any questions about using GUIs, I would recommend that you keep posting questions on SH and they will be answered as quickly as possible.

If you would like to figure out how to make a GUI, I would recommend visiting Roblox's official development website's post about GUIs: https://developer.roblox.com/en-us/articles/Intro-to-GUIs