This is the 8020th most frequent Russian word.
красная
"Красная" means "red" (feminine form) in English.
Here, "красная" is used as an adjective describing the book, denoting its name and emphasis as 'red.'
Красная книга содержит информацию о редких животных.
The Red Book contains information about rare animals.
In this case, "красная" functions as an adjective describing the pen, specifying its color.
На столе лежала красная ручка.
A red pen was lying on the table.
Here, "красную" is the feminine direct object form of "красный," describing 'закат' (sunset), referring to its red color due to the time of day.