The black veil that Rev. Mr. Hooper refuses to remove is the main symbol in this story. It is symbolic of many things. It is symbolic of death-which is happening to Hooper. It is also symbolic of mourning, possibly the mourning of the sins of mankind. Hooper himself explains that the black veil is symbolic of mankind's secrets that are hidden. There are many ways that the veil could be seen as symbolic.
Critics have suggested that the veil is symbolic of Original Sin or Hooper's pride.