At this point in the story, the narrator, Mama, is speaking about her interactions with white men. As an African American woman raised during times of racial turmoil and violence, Mama would doubtless have experienced racist abuse and would likely have been conditioned by experience to be wary of white men.

Therefore, when she says that she has always talked to them (meaning white men) "with one foot raised in flight" and her "head fumed in whichever way [was] farthest from them," she means that, whenever she has spoken with white men, she has always been ready to run. She has probably not literally had one foot raised and her head turned in the direction of her possible escape, but she has always, nonetheless, known that she must be ready to run should the interaction turn violent or abusive. The implication is that as a black woman she has always felt like prey, at the whim of her predators, and the most dangerous of these predators has been the white man.