It is a metaphor, and a rich one at that.
“Putting it negatively, the myth of eternal return states that a life which disappears once and for all, which does not return, is like a shadow, without weight, dead in advance, and whether it was horrible, beautiful, or sublime, its horror, sublimity, and beauty mean nothing.” - Milan Kundera, The Unbearable Lighteness Of Being
Think about this for a second. If your life disappears immediately after it ends, and more rigorously, if each second of your life disappears immediately after it ends, then everything you do is inherently meaningless. Important Sidebar: ( Kundera argues (and I agree) that this meaningless isn't necessarily negative, but instead allows for one to enjoy life without adding the "unbearable burdern" ).
If nothing we do repeats, we - in a certain way - don't exist. We are in constant flux and have no permanence. We are not bound to the significance of our own ego, because it is weightless. Consider the shame suffered by a Victorian Era woman who got pregnant out of wedlock. For her that shame was likely a crushing burden filled with suffering. How important is it now? Since her life doesn't return, the shame is gone, forever dissipated into nothingness, affecting no one.
Now - the opposite:
"If every second of our lives recurs an infinite number of times, we are nailed to eternity as Jesus Christ was nailed to the cross. It is a terrifying prospect. In the world of eternal return the weight of unbearable responsibility lies heavy on every move we make. That is why Nietzsche called the idea of eternal return the heaviest of burdens." - Kundera
If each action we take; each second of our existence; each thought is to recur infinitely, we are forever bound to the consequences of our actions. There is an unbearable amount of heaviness and seriousness associated with all things. Life cannot be taken lightly, because the weight of our actions will be with us infinitely.
The true spirit of Nietzsche's metaphor, at least as he intended, is:
“My formula for greatness in a human being is amor fati: that one wants nothing to be different, not forward, not backward, not in all eternity. Not merely bear what is necessary, still less conceal it—all idealism is mendaciousness in the face of what is necessary—but love it" - Nietzsche
Nietzsche was not a nihilist. His outlook can be looked at as somewhat of a mixture of the polar viewpoints. The idea of amor fati (love your fate) is much easier without eternal return, because all fate is etherial and meaningless in that context. However, what Nietzsche says in "The Gay Science" is:
"What if a demon were to creep after you one night, in your loneliest loneliness, and say, ‘This life which you live must be lived by you once again and innumerable times more; and every pain and joy and thought and sigh must come again to you, all in the same sequence?'"
Thus, he is not asserting that eternal return is the case, but he is asking, 'what if it is the case?'.
Then he goes on to say:
"Would you throw yourself down and gnash your teeth and curse that demon? Or would you answer, ‘Never have I heard anything more divine’?"
In other words - if this life is to recur eternally, 'Will I love this fate?' or 'will I despise this curse?'.
Thus, I must live my life in such a manner as if the events I experience were to be my fate eternally, and in so doing, I will create a life worthy of living eternally.