What it is about, Wikipedia says it clearly enough: O’Neil, a mathematician, analyses how the use of big data and algorithms in a variety of fields, including insurance, advertising, education, and policing, can lead to decisions that harm the poor, reinforce racism, and amplify inequality.

My 2 cents: it is a must must read for people working in the fields of big data, algorithms, machine learning and AI, to learn to take into account the social impact of the systems they are building. Or ambiguously, take care they deliver good systems.