It isn't really too much of a novel because it has a very thin plot with very little drama or tension. It is more like a deep reflection. But it is very good.
The book's perspective on human condition is very cynical. My understanding, when I read it, is that by "unbearable lightness" what the author means is that we are absolutely irrelevant and the whole idea of "meaning of life" is just pathetic.
Yeah, I know: I sound like an atheistic teenager. But, please, don't get fooled. Kundera wrote a billion times better than me. He deserves to be read, even if I sound childish.
If you like Albert Camus you'll love Kundera (and vice-versa).