The Scarlet Letter is the title of a book by Nathaniel Hawthorne. It tells the story of Hester Prynne and her daughter Pearl. Hester had an affair with the minister Arthur Dimmesdale. Her husband, Roger Chillingworth, has been away for a few years. He returns, discovers his wife's indiscretions and vows to find out who the adulterous man is. Chillingworth figures it out, Dimmesdale confesses, dies, and Chillingworth dies soon after too.
The title The Scarlet Letter is important because Hester was forced to wear a scarlet "A" on her clothing. It was meant to identify her as an adulterer. And for many years, it did just that. But Hester's hard work in the town, and her care for the poor and the sick caused the townspeople to associate the letter "A" with a different word. They began to think of the "A" as meaning "able."
Why Hawthorne chose to title his book The Scarlet Letter is a mystery. It's a good title though, because it focuses on how the letter itself caused a shift in public perception of its wearer. The story is a transformation story of Hester for sure, but that's not the biggest transformation that happened. The town and people in it were most transformed by Hester and her scarlet letter. The letter is an important catalyst to those changes and an important symbol of Hester and her work.