Hidden is both a property and a dice entry; the two are always found together and only in exploration scenarios. Challenges may have Hidden entries, as may region exits and other things such as treasure or devices.
PCs must Search to find something that is Hidden within their occupied region. If they do, anything Hidden uses the dice provided by its Hidden entry to oppose the roll. Ties are in favor of Search.
Challenges with Hidden entries are usually revealed if they play an Action, particularly Traps. Some Creatures may be able to stay Hidden even after playing Actions against the PCs. Either the Creature entry, the scenario writeup, or the DM determines this, with the DM having the final word.
A Challenge that is Hidden conveys the usual visibility penalties on targets regardless of lighting conditions or other considerations.
For more information, see Player's Guide Chapter 5: Exploration.