This is the 878th most frequent Ukrainian word.
усі
"Uсі" means "all" or "everyone" in English.
Here, 'усі' is a pronoun used as an adjective to mean 'all', describing the noun 'книги' (books).
Усі книги на полиці дуже цікаві.
All the books on the shelf are very interesting.
In this sentence, 'усі' is used as a pronoun to mean 'everyone', referring to a group of people.
Усі прийшли на час.
Everyone arrived on time.
Here, 'усім' is the dative plural of 'усі', used to mean 'to everyone' in the context of expressing gratitude.