This is the 13769th most frequent Russian word.
нижегородская
"Нижегородская" means "pertaining to Nizhny Novgorod," a city in Russia.
Here, "Нижегородскую" is used as an adjective modifying "историю" (history) to specify its geographical context.
Я изучаю Нижегородскую историю, чтобы лучше понять культуру России.
I am studying Nizhny Novgorod history to better understand Russian culture.
In this sentence, "Нижегородская" characterizes "ярмарка" (fair) to indicate its association with Nizhny Novgorod.
Нижегородская ярмарка была известна своим обилием товаров.
The Nizhny Novgorod fair was known for its abundance of goods.
This time, "нижегородские" is an adjective describing "сувениры" (souvenirs), indicating that the souvenirs originate from Nizhny Novgorod.