This is the 2060th most frequent Catalan word.
diu
"Says."
In this sentence, 'diu' is the conjugated form in the present tense of the verb 'dir' (to say) for the third person singular, used to report speech.
Ella diu que el llibre és interessant.
She says that the book is interesting.
Here, 'diu' is used metaphorically for an object (the sign) providing information, representing an extension of speech.
El cartell diu que l'oficina està tancada.
The sign says that the office is closed.
In this case, 'diu' represents conveying specific, potentially verbal or written, information.