At the beginning of Passing, Irene feels faint and goes into a whites-only hotel to drink a glass of iced tea. Irene's skin is sufficiently light that no one questions her presence there, and she reflects on how absurd it is that white people always believe they can distinguish between Black and white people. Their failure to do so makes a mockery of such segregated spaces as the one in which she is sitting.
White supremacy and segregation are based on the idea that Black people are recognizably inferior. If no one can in fact tell whether someone is Black or white, then this cannot be true. The act of passing therefore makes it clear that racial discrimination is based on a lie.
The most extreme example of the phenomenon of passing is found in Clare, who has not merely pretended to be white for an hour in order to drink some iced tea at a hotel, but has changed her life, permanently entering white society without any of the people around her questioning her racial origins. Even her husband, John, does not realize that he has married a Black woman. John is virulently racist and calls his wife "Nig" because her skin is darker than hers. Despite this, it does not occur to him that his wife is anything other than a white woman with somewhat darker skin than usual. The act of passing shows the folly of John's racism, and that of the society he represents, by demonstrating how easily they are duped.