Like many Quakers in colonial America, Hannah Tupper and her husband Thomas were branded on their foreheads because of their faith. The brand is an outward symbol of the Tupper’s outcast status. (Kind of like Hester Prynne’s scarlet “A” in The Scarlet Letter.) It is also suggests the cruelty and prejudices peoples of different faiths faced in the colonies:
“But in the town there was not an inch of land to spare, not after they’d seen the brand on our foreheads.” (9.60)
The brand is a physical reminder of Hannah’s religious difference – one with very real consequences. The people of Wethersfield will not allow her to own land in town after they’ve seen the brand on her forehead.