The previous posters covered the concept with regards to this specific short story, so I will discuss in general. Subject matter is what the story is about. Plot can fall under this category, as can motifs (ideas or concepts which are present throughout a work of literature). Motifs are often confused with themes today, even in academia. However, a theme is a universal message about life or human nature implied by a text. One way to distinguish between theme & subject matter is to first ask "What is this piece about?" Then you will know the subject matter. For example, in "The Masque of the Red Death", we have a story about a devastating plague, & its effects on a seemingly immune group. Once you have determined this, ask yourself, "What does this text say about the subject matter?" Then you will know the theme. Again, in "The Masque of the Red Death", Poe is suggesting that we as humans cannot escape our ultimate fate. A good way to judge if you're accurately stating theme is to make sure it's always a complete sentence. Otherwise, it cannot be a theme.
The Masque of the Red Death, published in 1842 by Poe is about Prince Prospero's attempts to avoid a deadly plague by the name of Red Death. He hides in his Abbey.The Prince and other wealthy noblemen have a masquerade ball in seven rooms of his abbey--each room is a different color. A strange figure enters the party and the rooms. When the Prince finds him and asks who he is, he falls dead. The story is an allegory of the inevitability of death, representative of Gothic fiction.
The disease of the Red Death is purely fiction. The disease was inspired by either tuberculosis--which Poe's wife had--or cholera which reached epidemic proportions in Baltimore Maryland in 1831. Some critics think it was inspired by the Bubonic plague or the Black Death. The subject matter is the "Red Death" and escaping it. On a symbolic level, however, it is about trying to escape death which cannot be done.
Poe's story, The Mask(Masque) of the Red Death is generally interpreted as an allegory in the Gothic tradition. Prince Prospero( the name obviously reminiscent of Shakespeare's magician-duke) and the nobles of his Court take shelter in the walled Abbey to fend off the mortal plague, called the Red Death. They seem rather indifferent to the sufferings and agony of the killer disease, and spend their time in all luxuries, waiting for the plague to disappear. But the prince can not escape death as he falls dead in the Black Room(one of the seven coloured rooms) during the masquerade ball,having faced a Black stranger, presumably representing death.
The subject of Poe's story is that nobody can escape death . Using the genre of a Gothic thriller, Poe has worked out a tale of mystery and suspense with a moral underlying it. The story was written at a time when Poe's wife was suffering from consumption, a disease that Poe's mother also died of.