The mousemove event
The mousemove event notifies your handler when a mouse moves over a particular element. You set up your handler using the element’s onmousemove property. Once you’ve done that you’ll be passed an event object that provides these properties:
clientX, clientY: the x (and y) position in pixels of your mouse ...
What does it mean by setting up the handler? A tip: to set up a handler means to assign the handler/function to a property of the target.