This is the 2352nd most frequent Russian word.
неизвестный
Unknown.
Here, 'неизвестный' translates to 'unknown' and describes someone who is unfamiliar.
На улице появился неизвестный человек.
An unknown person appeared on the street.
In this case, 'неизвестном' is the form of 'неизвестный' used in the prepositional case to describe an unspecified or unfamiliar place.
Эта история произошла в неизвестном месте.
This story took place in an unknown location.
Here, 'неизвестно' is used in the neuter form as a predicate to express something that is not known or understood.