What's the meaning of butterfly in the Antebellum poster?

Though butterfly's meanings can vary in different cultures, all of them are associated with the rebirth and change. The change from a caterpillar to a beautiful butterfly represents a positive transformation. The movie tends to hint on a powerful change that is long overdue.

From this article of Screenrant (emphasis mine),

Butterflies mean different things to different culture and religions, but they are commonly associated with change and rebirth. They tend to represent a deeply personal and positive transformation — the imagery of a caterpillar emerging into a beautiful butterfly is a common one used to depict this powerful symbolism. Antebellum utilizes the butterfly not to represent a powerful change that has already occurred, but one that is long overdue. The change in question is the racism deeply ingrained in America's history.

Continuing from that article,

The ending of Antebellum comes full circle with the Faulkner quote that opens the movie. Things may have changed, but there's still plenty of work to be done. Just because there have been some milestones passed in America's history in regard to racism doesn't mean racism is dead. Antebellum's ending shows that traces of the ugly past are still very much present in modern-day culture. Antebellum's use of butterflies is a subtle stand against that. The United States needs to find a way to undergo a deep and personal rebirth so the country can emerge on the other side as something beautiful.

From Essence,

“The butterfly tattoo represented that Julia, Kiersey [Clemons]’s character, was a disciple, a believer, a follower of that doctrine of Veronica Henley’s, of the coping persona. So when she says, ‘I know you. You’re the one who can get us out of here. Why are you acting like this?’ it’s because she knows her.” And on the cover of Veronica’s book is a butterfly, representing change or a metamorphosis.