Hidden models can choose to deploy "hidden" within another friendly ranked unit, either the unit it belongs to (determined in each army list) or in other separate units (the army book in question will state which units) – make a note of which unit is concealing the hidden model(s).
A hidden model is not placed on the table during deployment, but is revealed later during the game. If the concealing unit is wiped out or flees from the battlefield before the hidden model is revealed, the hidden model counts as a casualty. There is no other way a hidden model can be harmed before they are revealed.
Hidden models may be revealed at the beginning of any of your Movement phases, or at the start of any Close Combat phase. Declare that the unit contains a hidden model and place the model in the front rank of that unit, displacing models as you normally would if a character had joined the unit.
A model with this special rule cannot be your army General. Furthermore, other units can never use their Leadership value.