This is the 21st most frequent Spanish word.
los
"The" (definite article, plural, masculine or mixed gender).
Here, 'los' functions as a direct object pronoun referring to a group of masculine or mixed-gender individuals or objects.
Los vi ayer en el parque.
I saw them yesterday in the park.
In this sentence, 'los' is a definite article used with the plural masculine noun 'niños' (boys/children).
Los niños jugaron todo el día.
The boys/children played all day.
In this sentence, 'los' is a definite article used to refer to the plural masculine noun 'libros' (books).