It's very simple. Identity politics is the politics of identity, where an inherent, unchangeable characteristic (race, sex, etc.) is used to define a victim group and an oppressor group. This is inherently unhealthy. Feminism is the best example. Compare it to the politics of situation (not usually called that), where we focus societal attention and resources to assist people of any identity who are in a troublesome situation, such as poverty, unemployment or depression. This is healthy politics.