A graphical user interface (GUI) is like a picture of a computer program that you can touch and click. Testing it means checking to make sure that everything works the way it is supposed to when you touch and click it.
Imagine you have to put toys in a toy box. You need to make sure that you can open the lid, put the toys in, and close the lid without anything getting stuck or breaking. This is kind of what we need to do with a GUI - we need to make sure that all the buttons, menus, and other things you see on the screen work properly and do what they are supposed to do.
To do this, we use a special type of computer program that helps us test the GUI. It's kind of like a robot that sits in front of the computer and clicks all the buttons and types all the words that a person would normally do. It does this very quickly and can catch any mistakes or problems that might be hard for a human to notice.
When we find a problem with the GUI, we tell the people who make the program about it so they can fix it. This way, when you use the program later on, everything will work the way it is supposed to, and you can play with your toys (or use your program) without any issues.