What does it mean by 'The hunter who chases two rabbits catches neither'?

If you try to accomplish two like things at the same time, you will fail, or do poorly, at both. So say for example you want to learn to play more than one musical instrument. Don't try to learn the piano and violin at the same time. First master the one most important to you, then move on to the other. Your chances for success will be much higher.