The Apocalypse is from the Greek (lifting of the veil) and usually applies to the Book of Revelation. It predicts the future, more specifically the final battle between angels and demons. This includes the battle between Christ and the "Beast" (Antichrist) at Armageddon. Both the Apocalypse and Armageddon are used metaphorically to mean the end of the Earth, or to reference Judgment Day.
The key to understanding the symbology of Revelation is to understand that the symbols used are from the bible. It is biblical symbology, and therefore anyone with a reasonable knowledge of the bible should be able to understand the symbology of Revelation. It is not an accident that Revelation is the last book in the bible because it derives its symbology from all of the previous books, both from the Old and New Testament.
Most of us coming to Revelation will find its symbology obscure, we are simply not used to this type of language. But when we come to "a Lamb standing, as though it had been slain" (Rev 5:6), we ought to be able to figure out that this is Jesus. Based on "Behold, the Lamb of God, who takes away the sin of the world" (John 1:29) and "like a lamb that is led to the slaughter" (Isa 53:7). This should be quite familiar to us, so we know that the Lamb refers to Jesus (30 times in Revelation). There is always the temptation to take Revelation literally rather than symbolically, but this gets us into trouble, if we impose modern imagery for the biblical symbolism of Revelation. What I want to show is that the symbolism of Revelation is biblical symbolism and how this helps us make sense of the book.
We should get the idea that Revelation is a symbolic book because there are certain symbols that are explained within Revelation itself. There are at least eleven verses where Revelation explains its own symbology.