Kitsch gets a lot of attention in The Unbearable Lightness of Being, so you should make sure you understand the basic concept. Kitsch is a German word that's been adopted by a number of other languages, including English. It refers primarily to art that is overly sentimental or melodramatic, and so refers to aesthetics. What's interesting is the way Kundera uses the concept in his novel, not to talk about art, but to talk about political ideology.
To begin, Kundera asserts that kitsch is an aesthetic ideal "in which shit is denied and everyone acts as though it did not exist" (6.5.4). He's not just speaking literally here, but about all the bad, disgusting, negative, violent, depressing things in the world. "Kitsch excludes everything from its purview which is essentially unacceptable in human existence" (6.5.5).
Kundera then moves on to politics. "Kitsch is the aesthetic ideal of all politicians and all political parties and movements," he says (6.9.1). He gives the example of politicians kissing babies as the ultimate kitschy political move. When Sabina recalls the communist parades of her youth, she remembers that the parades tricked the participants into celebrating Communism by pretending they were celebrating life – a hokey, sentimental life embracing only the positive (see Part 6, Chapter 7).
According to Unbearable Lightness, this is actually not so bad in itself. The problem comes when you have to deal with totalitarian kitsch. He explains this in detail, so we'll let him do the talking here:
Those of us who live in a society where various political tendencies exist side by side and competing influences cancel or limit one another can manage more or less to escape the kitsch inquisition: the individual can preserve his individuality. The artist can create unusual works. But whenever a single political movement corners power, we find ourselves in the realm of totalitarian kitsch.
When I say "totalitarian," what I mean is that everything that infringes on kitsch must be banished for life: every display of individualism (because a deviation from the collective is a spit in the eye of the smiling brotherhood); every doubt (be-cause anyone who starts doubting details will end by doubting life itself); all irony (because in the realm of kitsch everything must be taken quite seriously). (6.9.2-3)
Go back to Sabina's cryptic thought about 200 pages earlier, that "behind Communism, Fascism, behind all occupations and invasions lurks a more basic, pervasive evil and that the image of that evil was a parade of people marching by with raised fists and shouting identical syllables in unison" (3.5.8). Now we know what that pervasive evil is: totalitarian kitsch.
So how does one fight kitsch? One answer has its roots in the original, artistic definition of kitsch as sentimental or hokey art. From this perspective, beauty is the enemy of kitsch. The other answer has its roots in the political definition of kitsch as forced conformity. In this sense, someone who insists on individuality is the enemy of kitsch. Sabina, who openly proclaims "My enemy is kitsch!", manages to do both (6.11.6). Jump to Sabina's "Character Analysis" to see how she pulls it off.