This means the disclosure of something that was unknown. There are two types of revelation: natural and special. Natural revelation is that which is revealed about God through what we can see in creation (Rom. 1:20). Through creation, we may learn that there is a God and that He is in control and that He has an order and that He is concerned for our welfare. However, through natural revelation, we are not able to discover the plan of salvation. That comes from special revelation.
Special revelation is that which is given to us through Prophets, the Bible, and even visions and dreams (Num. 12:6-8). The ultimate in revelation is the incarnation of Jesus because He came to reveal the Father to us (Matt. 11:27, Luke 10:22, Heb. 1:1-3) and to communicate to us the gospel (1 Cor. 15:1-4) by which comes salvation.