"Nobody knew my rose of the world but me [...] I had too much glory. They don't want glory like that in nobody's heart."
The epigraph is a line from a Tennessee Williams play called "The Rose Tattoo," which is, interestingly enough, about the difficult relationship between a mother and daughter. Sula is also about mothers and daughters to a large degree, so Williams's play seems like an appropriate source for the epigraph. Another interesting tidbit is that Morrison wrote her Master's thesis on William Faulkner, a southern writer like Williams who often deals with the issue of family—so there seems to be a connection here, too. Morrison is obviously familiar with how Southern writers deal with the messed-up things that families can do to a person.
And there's a lot we can unpack about the epigraph as it relates to the novel. The rose in both the epigraph and in the play's title could be linked to the birthmark on Sula's face that so many people think looks like a rose. Like a tattoo, the birthmark is an outward marker of her identity. But the idea that no one knows this rose except for Sula also makes sense since the people who live in the Bottom clearly can't see or deal with the inner "glory" that Sula knows she possesses.
So perhaps the epigraph refers to both the "rose" on her face and the "rose" that she alone knows is inside of her.