This is the 5934th most frequent Spanish word.
vacías
"Vacías" means "empty" (plural, feminine).
In this sentence, 'vacías' serves as an adjective describing the condition of the 'cajas' (boxes) being devoid of content or empty.
Las cajas están vacías.
The boxes are empty.
Here, 'vacías' acts as the conjugated second person singular (tú) form of the verb 'vaciar,' meaning 'to empty,' indicating action.
Nunca vacías tu mochila después del colegio.
You never empty your backpack after school.
In this usage, 'vacías' is functioning as an adjective describing 'palabras' (words), metaphorically expressing that they lack substance or sincerity.