Clicking this menu tells the clock that it should avoid application focus at all times. When this menu item is selected, a check  will appear next to it on the Clock Menu, and it will mean that you do not ever want the clock to become the active application unless you click on it. 

If you're typing a Word document, for instance, sometimes the clock will suddenly cause Word to lose focus. When this happens, whatever you type no longer goes into your Word document until you click on it again. This can be annoying, so we added this option. 

We strongly recommend turning this feature on: in fact, this is the default setting.