The Red Death is a terrible plague, a highly contagious disease that leaves a trail of death and suffering in its wake.
Fourteenth-century Europe was devastated by a similar disease called the bubonic plague, also known as the Black Death. It's estimated that something like sixty percent of Europe's population was wiped out by the plague, spread by rats' fleas.
If anything, the Red Death's even worse. It's caused massive devastation in Prince Prospero's dominions, leaving them severely depopulated. It's a truly horrible disease that suddenly attacks its victims, leading to sharp pains, sudden dizziness, and bleeding from every pore. (That's why it's called the "Red" Death). And to make matters worse, the plague leaves such big ugly blood stains all over your face and body that it frightens people away from helping you. Not that there's much they can do, of course, as the Red Death's completely incurable.
But Prince Prospero really couldn't care less. He doesn't give a hoot about the welfare of his people. He sees the Red Death as a great opportunity to have a good time, to throw a massive party for himself and his rich aristocratic friends inside the fortified walls of his castellated abbey. Here they'll be safe from the murderous pestilence raging outside. Or so they think.