This is the 12739th most frequent Russian word.
красную
"Красную" translates to "red" (feminine singular accusative form).
Here, 'красную' is a descriptive adjective in the accusative case modifying the noun 'рубашку' (shirt).
Я купил красную рубашку.
I bought a red shirt.
In this sentence, 'красную' refers to 'книгу' (book), used in the accusative case due to the verb 'ищем' (looking for).
Мы ищем красную книгу.
We are looking for a Red Book.
Here, 'красную' describes 'машину' (car), again in the accusative case as it is the object of the verb 'видел' (saw).