This is the 23rd most frequent Dutch word.
dat
"Dat" in Dutch means "that" in English.
Here, "dat" functions as a relative pronoun which refers to "het boek" and introduces the relative clause describing the book.
Het boek dat op de tafel ligt is van mij.
The book that is lying on the table is mine.
In this case, "dat" is used as a demonstrative pronoun to refer to a specific event or situation previously mentioned or understood from context.
Dat was een leuke dag!
That was a nice day!
In this sentence, "dat" is a subordinating conjunction used to introduce a subordinate clause that specifies the content of the main clause's thought or statement.