This is the 7th most frequent Spanish word.
las
"The" (feminine plural definite article).
In this sentence, 'las' functions as the feminine plural definite article, indicating that it modifies a noun (ventanas) in the feminine plural form.
Las ventanas están cerradas.
The windows are closed.
In this sentence, 'las' serves as a direct object pronoun in the feminine plural form, replacing a previously mentioned feminine plural noun.
No puedo verlas desde aquí.
I can't see them from here.
Here, 'las' is again the feminine plural definite article, accompanying the noun (lámparas) in the feminine plural form to specify it.