Hester Prynne's scarlet "A," as seen in Nathaniel Hawthorne's The Scarlet Letter, symbolizes many different things within the novel. As the novel progresses, the symbolism of the scarlet "A" does as well.
In the beginning, the "A" is meant to illustrate Prynne's adulterous affair. As her punishment, Prynne is forced to bear the scarlet letter as a symbol of her sin. The "A" stands for adultery.
Over time, the meaning of the letter changes. As Prynne proves her ability to face conflict, she proves (time and time again) her ableness to survive. Therefore, the scarlet letter's meaning changes. No longer a sign of adultery, the letter signals Prynne's ableness to survive.
Later in the novel, the meaning changes again. As the novel progresses the impact of the letter wears off. In fact, when seen by the Native Americans (in chapters 21 and 22), they believe her to be someone of importance because of the letter she bears.
Essentially, the letter symbolizes the ever-changing nature of symbols in society. What the letter was meant to signify (sin) does not withstand time. In fact, the symbol changes as the townspeople change (mentally about Prynne).