This is the 2100th most frequent Russian word.
первая
"Первая" means "first" (feminine form) in English.
In this sentence, 'первая' modifies the noun 'книга' and signifies its ordinal position as the first in a series.
Это первая книга, которую я прочитал.
This is the first book that I read.
Here, 'первой' is in the prepositional case and describes 'остановке', indicating the location referring to the initial stop.
На первой остановке мы вышли из автобуса.
At the first stop, we got off the bus.
In this sentence, 'первая' is used as a predicate adjective agreeing with the subject 'она', specifying ordinal position in the context of arrival.