This is the 7909th most frequent Dutch word.
dàt
The Dutch word "dàt" means "that" (used for emphasis or to specify).
'Dat' introduces a subordinate clause.
Ik weet dat hij vandaag komt.
I know that he is coming today.
The first 'dat' is the article, and the second 'dat' refers to the noun 'book' in the sentence.
Het is dat boek dat ik heb gelezen.
It is that book that I have read.
'Dat' is used as a demonstrative pronoun referring to something near the speaker.