"Doom" means "disaster that can't be avoided," "a tragic fate," "an inevitable bad destiny." Often, the context is "doomed to die." It once was used very seriously. To modern ears it sounds dated, old-fashioned, and melodramatic, and it is often used sarcastically, to mock somebody's pessimistic prediction.
Some serious uses:
"Sir, I see by the book in my hand that death is my doom, and that I am then to meet my Judge..."--Bunyan, "Pilgrim's Progress"
"'Here is a nation,' goes on Wainwright, 'on the brink of destruction. A man of intelligence could rescue it from its impending doom in one day by issuing the necessary edicts and orders...'"--O. Henry
"I am thy father’s spirit,/Doom’d for a certain term to walk the night..."--The ghost, in Shakespeare's "Hamlet"
"Alas, regardless of their doom,/The little victims play!/No sense have they of ills to come,Nor care beyond today." Thomas Gray
"their day of disaster is near and their doom rushes upon them."--Bible, Deuteronomy 32:35
Here's an example of modern use as mockery, in this case mockery of a NASA study: "NASA Study Concludes We’re Doomed! Well, comrades, the end is nearer than we thought. We’re doomed. Yes, that’s what I said. Doomed! Our civilization, or what’s left of it, is on the fast track to extinction. (or worse)..."