Tomas always tries to convince Tereza that love and sex are different things, and she tries to think about this as she flirts with strangers at the restaurant where she works. Flirting, the narrator says, “is a promise of sexual intercourse without a guarantee,” and Tereza is very bad at it. Tereza wishes that she could learn to be “light” because she knows she’s a burden to Tomas, but by taking flirting so seriously, she fails at it.
Tereza’s flirting represents her attempt to be “light” like Tomas. She wants to discover if there can be sex without love, and flirting offers her a safe way of testing this theory, since there is no “guarantee” of sex. But Tereza can’t even flirt well, which, Kundera implies, is because she is too heavy.