This is the 3149th most frequent Russian word.
любую
"Любую" means "any" (feminine, accusative case).
In this sentence, 'любую' again functions as an adjective in the accusative case, modifying 'ручку' (pen) in gender and case to indicate an unspecified, interchangeable choice.
Принеси мне любую ручку, пожалуйста.
Bring me any pen, please.
The term 'любую' in this example is used as an adjective in the accusative case, aligned with 'тему' (topic), referring to an unrestricted choice within possible options.
Ты можешь задать любую тему для обсуждения.
You can propose any topic for discussion.
Here, 'любую' is an adjective in the accusative case, meaning 'any', agreeing with 'книгу' (book) in gender and case.