This is the 600th most frequent Spanish word.
aquellos
"Those" (referring to masculine or mixed plural nouns far from the speaker).
In this sentence, 'aquellos' is used as a demonstrative adjective to refer to 'libros' (books). It indicates specifically the ones in the context, which are farther away in distance or time.
Aquellos libros en la estantería son de mi abuela.
Those books on the shelf belong to my grandmother.
Here, 'aquellos' is used to denote a time in the past, emphasizing a specific period distinctly separated from the present.
Recuerdo aquellos días felices de mi juventud.
I remember those happy days of my youth.
In this sentence, 'aquellos' is used as a pronoun to replace a noun, referring to people specified in the context who are distinct from the current situation.