This is the 12th most frequent Dutch word.
het
"The" (definite article).
Here, 'het' functions as a pronoun referring to a previously mentioned object.
Ik heb het nog niet gezien.
I haven't seen it yet.
In this sentence, 'het' serves as an impersonal subject, introducing the clause.
Het is belangrijk om te leren.
It is important to learn.
In this sentence, 'het' is used as the definite article for a singular neuter noun 'boek'.