From the Saga I Rulebook:


Some characters are able to use the Special ability Hide. While they are hidden, they cannot be the object of any hand-to-hand attack, ranged attack, effect, or interception; simply put, enemy characters are considered not to be aware of their presence. Other players cannot declare any attack or Special Ability against the hidden character. When a hidden character makes an attack or performs any offensive action, the character is revealed and loses the state of hidden. It is also possible to detect a hidden character by using the Seek ability.
A character cannot hide if it is already in the Zone of Control of an enemy character.

From the Web Addendum Vol. I:

Stealth and Hidden Units

Some characters are masters at hiding themselves and moving in the shadows. This is represented in the game by the ability Stealth. This is a Subterfuge Special Ability that gives the State of Hidden, by which a Character can’t be the target of Hand-to-Hand or Ranged Attacks, or other harmful effects. Enemy units are considered to be unaware of its presence, and therefore unable to act against it. A rival can’t declare attacks or use Special Abilities against a Character while it is Hidden. Area attacks do affect a Hidden unit, but only through indirect impacts from attacks or abilities targeting other miniatures close by. A Character cannot activate the ability Stealth if it is within an enemy unit’s Zone of Control.
The ability Stealth has Upkeep, which means a Character can stay hidden during future turns and continue its benefits. Only when a Hidden unit makes an Attack or other Offensive Action does it become visible and lose the benefits of this State. This is not applied to Special Abilities that only cause Effects, like Takanosuke’s Void Eye.
Hidden characters don’t affect Line of Sight, nor are they in the way when making movements or a charge through their position. Nothing stops them from intercepting a Ranged Attack or Charge, but they lose the condition of Hidden if they do so, as when they use any kind of attack. While a Character is Hidden, it cannot engage enemies in combat, so its enemies are free to move with no need to use an Escape Action in combat.
Hidden units can be detected with the Seek ability, as described in Basic Abilities.