The word "apocalypse" literally means a revelation. A "revelation" usually means a prophetic revelation, in an inspired dream or vision, about the end of the world.
In Western culture, the word is most commonly used about several books of the Bible, notably the Book of Daniel in the Old Testament and the Book of Revelation in the New Testament.
The Book of Revelation in the New Testament contains dramatic, colorful, terrifying accounts of the "End Times," including angels, celestial wonders, the "sealing" of the 144,000 who will be saved, and the final battle between Good and Evil at Armageddon.
If you have seen the movie "Raiders of the Lost Ark," the climactic scene where the ark is opened, is very "apocalyptic."
It is often used in a half-joking, exaggerated way. To say something is "apocalyptic" means "like the end of the world, as described in the Book of Revelation."