Setting priorities… and leaving everything else aside

What does setting priorities mean to you? Does it mean, knowing what things to do first? Or does it help you assess what to invest your time in in the first place? Sometimes you need to invest a lot of energy doing something; maybe not perfectly but well enough, definitely. But sometimes, it means investing your energy into one thing instead of the other. And sometimes... it means not doing something at all, or does it? How do you set your priorities?