If a creature is Hidden, you don't know what space they occupy, and you're Off-Guard to them.
Targeting a Hidden creature is difficult. If you suspect there's a creature around, you can pick a space and attempt an attack. You suffer double burden on your power roll (as if the creature was Concealed), but the GM makes your attack in secret. You don't know if you picked the wrong space to target. Regardless, you also don't know the effect's outcome.
↳ Using the Seek basic ability can help you find a Hidden creature, usually making them Observed or Concealed from you instead of Hidden.
If a creature is Hidden, that doesn't necessarily mean you're unaware of their presence. You might suspect a Hidden creature is in the room with you, even though you're unable to find their space.