This is the 9069th most frequent Russian word.
русская
"Русская" means "Russian" (feminine form).
In this context, 'русская' is an adjective describing 'книга,' emphasizing its origin or linguistic nature.
Это русская книга.
This is a Russian book.
Here, 'русская' serves as a predicate noun in a sentence identifying the nationality of the subject.
Моя подруга — русская.
My friend is Russian.
In this case, 'русская' is used as an adjective modifying 'песня,' indicating the cultural or linguistic identity of the song.