When we write a letter/email, we can talk about things that:
• happened (past),
• things that are happening (present)
• things that will PERHAPS happen (future).
With regard to the future we have little/no certainty. Not so with God: he knows the future! We don’t!
According to Revelation 1:1, God can reveal the future (apocalypse - unveil, uncover, pull the curtain). He can tell in advance things that must take place. When he does that, the person to whom he gives the revelation is called a prophet or a prophetess (in this case John).
The information the prophet receives and shares from God is called prophecy.