The idea of literary polyphony, originally a music term, was developed by the Russian critic Mikhail Bakhtin in his book Problems of Dostoevsky's Poetics, which should be consulted for a fuller understanding of a complex theory. Bakhtin summarizes it: "The essence of polyphony lies in the fact that the voices remain independent, as such, are combined in a unity of a higher order than homophony." For Bakhtin, it was the novels of Dostoevsky, in which various conflicting philosophies and ideologies were presented, who best exemplified his ideas.
As applied to Czech writer Milan Kundera's 1984 masterpiece, The Unbearable Lightness of Being, polyphony is useful but also problematic. The question asks what Kundera's "idea of polyphony is," but it is unclear if Kundera is aware of Bakhtin's theory, and, if he is, whether he is putting it into practice. What is striking about Kundera's novel is that he, the author, is part of the story. This makes it a better example of a post-modern novel or metafiction. He addresses the reader in part 2: "It would be senseless for the author to try and convince the reader that his characters once actually lived."
So, in some sense, the real voice in this novel is that of Kundera and, by drawing attention to himself as the creator of a fiction, he is speaking through his characters, an approach that would seem to be at odds with Dostoevsk, who remained a detached narrator. Unbearable Lightness can, perhaps, be better understood as a a novel that is in dialogue (Bakhtin has another major work called The Dialogic Imagination.) with other works, other writers, and with itself. Kundera is writing about history, politics, philosophy (Nietzsche in particular), and music, among other topics. But it's the author's (Kundera's) voice that is the dominant one.