In the book, it's used to describe the sound of a knock on a door. However, Joyce doesn't give it any real meaning beyond that.
Here's why it's important:
* It's a great example of onomatopoeia: The word is designed to sound like a knocking sound.
* It reflects Joyce's style: Joyce was known for using made-up words and playing with language. "Tattarrattat" is a perfect example of this.
* It's become a popular literary reference: The word has been used in other works of literature and is recognized even by people who haven't read *Ulysses*.
So, "tattarrattat" doesn't have a specific meaning in the traditional sense, but it's a powerful and memorable sound that embodies a certain kind of literary playfulness.