This saying basically means that you should make the best of what you get out of life. This saying has a lot to do with what kind of attitude you have in life. If something goes wrong then change your plans or if you are having a rough time it will make you feel a lot better to make the best of the situation or the best of what you have, even if it is not what you want.
If life throws undesireable things at you learn how to make those things desireable. This is a way of living that will cut down on stress and promote general happiness.