Cathy O'Neil is an academic mathematician who has also worked in the big data and finance industries. Weapons of Math Destruction is a critique of the way in which algorithms are used in a wide variety of fields, including finance, insurance, policing, and education, to make decisions which crucially affect people's lives. The people affected by these decisions generally do not know that the algorithms are being used, and even the organizations which use them do not know how they achieve their results, meaning that these decisions are never questioned.
The main argument in Weapons of Math Destruction is that the use of impersonal, opaque algorithms reinforces structural inequality in society in a way that may well be more intractable than personal prejudice. Fifty or sixty years ago, a person applying for a mortgage would have done so in person, probably speaking to a bank employee they already knew. This employee could have been racist and less willing to grant mortgages to Black families. However, what happens now is that an algorithm makes the decision, taking into account many factors which may well be racially specific, such as one's zip code. This decision is not seen as racist, since it is not personal, but it is rigid, unappealable, and discriminates against people of color. The same problem applies in the criminal justice system, college admissions, and many other areas.