To be Hidden, you must first successfully use the
Hide ability.
This means the character must have some type of obscurement to ALL hostile creatures in the encounter (as creatures will communicate if they see a creature trying to hide).
This status effect ( Hidden) is also a
Hidden Roll to the
Caster, this prevents metagaming, as the creature does not know if it is truly hidden or not.
If successfully Hidden,
Attack Roll the creature does, is considered at
Advantage.