This is the 12227th most frequent German word.
verbotene
"Verbotene" means "forbidden" or "prohibited" in English.
Here, 'verbotene' is used as an adjective meaning 'forbidden', describing the type of relationship.
Er hatte eine verbotene Beziehung.
He had a forbidden relationship.
'Verbotene' modifies 'Handlung', expressing that this action is not allowed.
Das Betreten des Grundstücks ist verbotene Handlung.
Entering the property is a forbidden action.
In this case, 'verbotene' describes 'Zone', indicating it is restricted or not permitted.