Kundera's title of the work reflects a passage in which he sought to distinguish between "weight" and "lightness." When he speaks of weight, he is speaking of the weight of our experiences, our memories. This weight defines who we are, what we do, and how we proceed. It is brought on by our choices in these domains. It is a part of us, but it is also crushing. We are pinned underneath it, as he says, and it is painful at times because weight usually is. At the opposite polarity is the notion of lightness, where we transcend our experiences and seek an "idyll where nightingales sing," (to paraphrase from another Kundera work, The Book of Laughter and Forgetting.) Lightness is a relinquishment of our weights, our experiences, our memories and is a state of beings where we are free to float for our choices no longer matter. The problem here, though, is that when something floats and glides it has no control over where it goes, where it ascends. This direction can only be given with weight. This creates a dilemma: The pain, crushing and agonizingly brutal, of weight pins us down and seeks us to covet or desire lightness. Yet, the absolutely free and detached nature of lightness is "unbearable" because the elements that truly defined us are gone, never to return. Hence, the unbearable lightness of being is the situation where we are no longer tethered to our identities, our sense of self, and we float without a sense of purpose or direction. Kundera forces us to choose with his metaphor, and finds our choices to be brutal regardless.
In the book, The Unbearable Lightness of Being, he explains this situation regarding his protagonist who is involved with a woman outside his marraige. The metaphor also has political implications with Soviet style Communist rule, and the notion of a shared community set against radical individualism. Both realms leave something to be desired, as both suffer from weight and lightness .