This is the 26th most frequent Dutch word.
dit
"This."
The word 'dit' is used here as a demonstrative pronoun to indicate a specific item near the speaker, which is 'boek' (book). It corresponds to 'this' in English and shows possession.
Dit boek is van mij.
This book is mine.
In this case, 'dit' is used as a demonstrative pronoun in a more abstract sense, referring to the concept or object being discussed, which is 'interesting idea'. It serves to highlight a specific idea being referred to in the context of the conversation.
Dit is een interessant idee.
This is an interesting idea.
Here, 'dit' is again a demonstrative pronoun, used to ask about the origin of a specific, possibly tangible or conceptual, item or idea.