A pressure plate is one component of a trap. It is normally hidden from view until discovered by searching or it is triggered. It will be triggered when you step onto it, a monster steps onto it, or an item is thrown onto it. A pressure plate will almost always trigger a trap, commonly causing harmful gas to flood the area from nearby floor vents. Therefore, if you discover a hidden floor vent a pressure plate can't be too far away.
There is one key room that is solved by throwing an item onto a pressure plate. This is the only instance in which a pressure plate does not trigger a trap.
Pressure plates can occasionally be hidden from the players view by flames , nets , bloodwort pods (maybe other stuff). This is a limitation of Brogue's display capacity. If the pressure plate is know to the Rogue, the player will be given an "Are you sure? Yes/No" prompt before accidentally stepping on the plate and triggering the trap (again).