GUIs are Graphical User Interfaces. That is what the acronym is but what is it really. You may have heard people say computers now use GUIs
The interface is the part of the program that shows things to you on the screen and that you often also enter information into. Originally this was all character based (those that remember DOS or those that use LINUX today will have used character interfaces. You were shown things only in text, no pictures, colors (or everything was a color like on IBM PCs used a green text) and you typed in text.
With a GUI pictures and colors are art of the screen. Windows uses a GUI and was first program that was popular that used a GUI. Actually, the MAC used a GUI before and Xerox Palo Alto lab actually created the GUI that Steve Jobs and Bill Gates copied.
The GUI looks more real life. It will have pictures to show us stuff. Newspapers today use a GUI approach. If you look at news stories on the Internet, they are using a GUI. They have more than text there, they also have pictures and maybe videos and may put some stuff in color. Twitter may be one of the few items that is very limited on using a GUI and tends to be more CLII (Command Line Interface) as basically all you can enter is words. They say graphics coming soon, but that has been their story several years. Instagram Takes the approach of a GUI to extreme on graphical as it is picture based and just now going to more text in it.
Pages that use GUIs look much friendlier and usually are easier to use. However, you will still find pages and sites on computers that just ask questions or present information all in text form.