Easiest way to think of a focus is that it is a 'saved target'. My first experience with using focus was for shackle spells (some use it for sheeping). I created a macro that when I right clicked on it, it set my current target, a mob as my focus. Later on, left clicking on it shackled the mob. I could then target my tank, heal him over and over again, pausing to click on the shackle button now and then to reshackle. I no longer had the mob targeted, but could still shackle him. I don't know if that helps or if I'm rambling, sorry.