A revealed tile, in fortress mode, is any tile that has a description when loo ed at.
All above ground tiles on the map start out revealed, as do all tiles above the surface. All subterranean tiles must be revealed by digging into them. All tiles adjacent to a revealed tile will also be revealed: you can see what the rock walls adjacent to a mined-out tile are made of, for instance, and you can see the outermost tile of each z-level of a mountain. You can also see the tile immediately below where a downward or up/down stairway has been placed -- this is because building (or digging) the stairway removes the floor between the levels. Upward stairways do not reveal the level above, because they do not remove the floor/ceiling above the stairs.
Underground features like caverns and magma pools start out hidden and must be discovered by miners. Upon breaching a feature, a message will be displayed informing the player of the discovery, and a chunk of the new feature "within view" of your dwarves will be revealed. See the Methods of exploration section of the caverns article for information on safely revealing more of the underground.