Es muss sein is an important concept in Unbearable Lightness, so you should make sure you're comfortable with it. The phrase is German and translates to, "It must be." Kundera tells you about the origins of the phrase as a motif in one of Beethoven's songs; you can read all about it in Part 5, Chapter 8. We'll talk about the importance of Beethoven in a second.
In this novel, the phrase comes up when Tomas is debating whether or not to return to Prague after Tereza has left him in Zurich. One he decides to follow her back, he tells his new boss: "es muss sein" (1.15.7). In other words: he has to follow Tereza back. It is his fate – he has no choice.
Now fate is an interesting concept in the context of the novel's themes of lightness and weight. If you've read "What's Up with the Title?" then you know all about Nietzsche's idea of eternal return. Hand in hand with eternal return is the concept of amor fati, or the love of one's fate. Remember that Nietzsche believed we could learn to embrace eternal return and the burden of weight that went with it. To do this is equivalent to embrace and to love one's fate. For the purposes of this novel, fate goes hand in hand with heaviness or weight. (Yes, this is a simplification of Nietzsche's ideas, but it's about as deep as we need to go right now.)
It's fitting that the phrase "es muss sein!" is associated with Beethoven, who in the narrator's mind, is a pretty weighty guy. At the end of the novel, Kundera refers to the man's "frown" and "improbably mane," and probably has this painting in mind. Not the most happy-go-lucky guy. On top of that, Beethoven is one of the great loves of Tereza, who, as we discuss in "Character Analysis," is associated with heaviness or weight. Tomas only learns to love Beethoven's music because of Tereza.
Now let's go back to Tomas. When we refer to Tomas's es muss sein, we mean the elements of his character or life that are beyond his control, that are imposed on him by fate. By telling the doctor "es muss sein" in regards to his return to Prague, Tomas claims that Tereza is part of his es muss sein. But is she?
This is a complicated question, and one that Tomas has a hard time figuring out for himself. At first he is certain that, yes, Tereza is part of his es muss sein – that's why he must return to Prague. But on the way back, he does some serious thinking. He thinks about all the fortuitous events that precipitated he and Tereza getting together. He identifies six, which is why Tereza becomes known as "the woman born of six fortuities" throughout the novel. This worries him considerably, since, if he and Tereza are together just by chance, they can't possibly be together by fate. Had any one of those six fortuities gone differently, he could be with some other woman instead. He comes to the "conclusion that the love story of his life exemplified not 'Es muss sein!' (It must be so), but rather 'Es konnte auch anders sein' (It could just as well be otherwise)" (1.17.5).
But that is not the end of the story. The narrator challenges Tomas's conclusion some time later. "Is not an event in fact more significant and noteworthy the greater the number of fortuities necessary to bring it about?" he asks (2.9.2). "Everything that occurs out of necessity, everything expected, repeated day in and day out, is mute. Only chance can speak to us" (2.9.2). This is an important line. What happens by fate = what happens by necessity = what is repeated = that which belongs to the realm of eternal return. What happens by chance = the fortuitous = what happens only once. And yet, says the narrator, what happens only once, by chance, can have meaning, too. This complicates the fundamental lightness/weight dichotomy of the novel, because it argues that what happens only once can have meaning.
As the novel moves forward, Tomas continues to struggle with the idea of fortuity and es muss sein. The narrator explores what belongs to Tomas's es muss sein later in the novel, in Part 5, and identifies two elements. First is Tomas's profession as a surgeon. "He had come to medicine not by coincidence or calculation," we learn, "but by a deep inner desire" (5.7.2). Second is Tomas's womanizing, "something of an 'es muss sein!' – an imperative enslaving him" (5.7.16). But still we wonder – where does Tereza fit in?
Tomas grapples with this question after a night of stomach pains and weird erotic dreams. After dreaming of the perfect woman – a woman who radiates calm and femininity – he decides that she is "the 'Es muss sein!' of his love" (5.23.4). And yet, if he had the choice between her and Tereza, he would choose Tereza, "the woman born of six laughable fortuities" (5.23.7). Tomas has no control over his sexual relationships with women – these belong to his es muss sein. But "love is our freedom," he decides. "Love lies beyond 'Es muss sein!'" (5.22.6).
You can start to see why it is an oversimplification to label characters in this novel as representing weight or lightness. On one level, Tereza represents heaviness for Tomas because she wants him to give up his philandering lifestyle and commit to her alone. On the other hand, she is born of fortuity and not of compulsion, and so for Tomas she belongs to the realm of lightness, not of heavy es muss sein. Your goal in reading and discussing this book should be to explore the way these different concepts manifest themselves in the different characters – not to box the characters in with one label or another.