Inside the computer, the mouse can be thought of as two numbers and a button. Where is the mouse and is it being clicked?
When you click, the computer tells a program where the mouse was when it clicked.
If the mouse was on something like a button, the program will follow its instructions on what to do when the button is clicked.
So, in your example, when you click your mouse on the back button, the computer tells the back button that it was clicked. The button then does what it was programmed to do, in this case, going back to the previous page.
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