This is the 17th most frequent Italian word.
gli
"The" (masculine plural definite article).
In this sentence, 'gli' is an indirect object pronoun meaning 'to him'.
Gli ho dato il libro.
I gave him the book.
In this sentence, 'gli' is the definite article used before certain masculine plural nouns that begin with a vowel or specific consonants.
Gli studenti sono nella biblioteca.
The students are in the library.
Here, 'gli' functions as the definite article for masculine plural nouns.