Let me provide one acutely small but important example of this: take a look at the 'pointer' that is controlled by the mouse (or track pad) and let's play around with it. Open a window in Finder for a folder or a hard drive. Move the pointer using your mouse (or track pad) to the oval button at the top-right corner of the window. Clicking this button would toggle the toolbar for the window. Try it a couple of times. Now very carefully, play around with the actual positioning of the point ON the oval button and figure out when it will actually toggle the window and when it won't.