these hips are big hips they need space to move around in. they don't fit into little petty places. (1-5)
The "petty places" the speaker references are a metaphor for the petty aspects of society which try to judge the speaker (and maybe even women in general) by fitting her into small, limiting stereotypes.
these hips are free hips. (6-7)
In many ways, this poem enacts the process of a woman taking control of her own body – and by extension, her own life. She's free, just like her hips are free.
they don't like to be held back. (8)
Clifton's making an oblique reference here to a rather long history of repression, when women had less access to education, jobs, and all sorts of opportunities for advancement.