GUI is an acronym for Graphical User Interface.
When you interact with a computer, you are called the user and the method by which the computer interacts with you is called an interface.
In the early days of computers, the interface was always text-based. The computer would display information in text, and you would issue commands to it in text.
Since the mid-1980's more computer interfaces have been designed to be graphical. This means boxes, pictures, lines, and a way of pointing to things on the screen with a mouse. Text is still important, but is not the only way to convey information or issue commands.