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In other words, you make snap judgements and assumptions about people based on their age, race, and sex.
Part of the thesis of that book is that you are a racist if you treat people as individuals. To be Anti-Racist, you must treat people as part of their collective and then treat them well or badly based on membership of that collective in the name of equity. Thus whites and asians must be treated poorly because of their race and other minorities treated positively because of their race.
Basically you fight racism with racial discrimination.