What does Milan Kundera mean by "the unbearable lightness of being?" themes about the novel

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,