This is the 56th most frequent Dutch word.
alle
"Alle" means "all" in English.
Here, 'alle' is an adjective modifying the noun 'kinderen', meaning 'all children'.
Alle kinderen spelen buiten.
All the children are playing outside.
In this sentence, 'alle' is an adjective qualifying the noun 'boeken', indicating 'every one of the books'.
Ik heb alle boeken gelezen.
I have read all the books.
In this usage, 'alle' functions as a pronoun meaning 'everyone' or 'all of them'.