This is the 2778th most frequent Russian word.
нагорного
"Of the highlands" or "upland."
Here, 'нагорного' is used as an adjective modifying 'хребта' (ridge), describing its characteristic as elevated or high.
На вершине нагорного хребта раскинулся густой лес.
On the top of the high ridge spread a dense forest.
In this sentence, 'нагорного' specifies a characteristic of the 'района' (area), indicating it is situated in the uplands.
Это был парк в нагорного района города.
It was a park in the upland area of the city.
Here, 'нагорного' describes 'плато' (plateau), giving it the property of being elevated or upland.