Most Windows programs are controlled by "messages", signals which keep it up-to-date with what’s happening (you’ve moved the mouse) or tell it what to do: close down, perhaps. If you’re thinking "so what?", then you may have a point. The typical PC user doesn’t have to know anything about this at all. If you’d like to automate common tasks via scripts or shortcuts, though, it may be worth looking at how this system can work for you.