It’s really simple to understand on one level; but on another level, it’s very hard to truly grasp and put in to practice. Here it is, stated very well by Rick Warren in The Purpose-Driven Church
The secret of effectiveness is to know what really counts, then do what really counts, and not worry about all the rest.
This makes sense if you think about it. Effectiveness means doing the right things. It doesn’t mean just getting things done, but getting the right things done.
If you are going to be effective, then, you need to know what the most important things are. But that’s not enough, because if you know what is most important but don’t actually do those things, it won’t help you. so you not only need to know what really counts, you have to actually do what counts.
But in seeking to do this, there are obstacles. There is a villain, so to speak: all the other things (many of which are good in themselves) which are outside of our core purpose and threaten to distract us from it by splintering our efforts and pulling us in too many directions. So in order to put first things first, you also need to know how not to worry about other things.
So: Know what counts, put it first, and know how to keep yourself from being distracted by everything else.