In apartheid-era South Africa, demographic groups were classified to delineate their legal rights and privileges under the stratified segregationist government. Whites were afforded the most privilege, and anyone classified as a different demographic group experienced varying degrees of subjugation. The four groups were "White," "Asian," "Coloured," and "Native/Black."
In Born a Crime, Trevor Noah explains why this system caused him particular strife (as well as how the book gets its name). His mother was a Black Xhosa woman, afforded the least privileges under the law. His father, by contrast, was a white European. Race-mixing was criminalized under the laws of apartheid, which meant Noah's very existence was evidence of a crime.
To avoid legal ramifications, Trevor and his mother had to keep his mixed-race status a secret. Instead, they pretended he was "Coloured," a separate racial distinction given to descendants of centuries-old intercontinental unions. Often, this meant they had to pretend they weren't even related—throughout much of his childhood, Trevor would either hide from view to avoid being seen with his mother or walk a few paces away from her with someone who more closely shared his skin tone.