This is the 21st most frequent Catalan word.
seu
"Seu" means "his," "her," "its," or "their" (possessive) or "seat/headquarters" depending on context.
In this sentence, 'seu' means 'headquarters', referring to the main office or official location of an entity.
Vaig visitar la seu de l'empresa.
I visited the headquarters of the company.
Here, 'seu' is used as a possessive adjective, meaning 'his' or 'her', to describe ownership of the textbook.
És el seu llibre de text per a la classe.
It is his/her textbook for the class.
Again, 'seu' is used as a possessive pronoun, indicating possession or association.