This is the 526th most frequent Persian word.
نیستند
"Are not"
Here, 'نیستند' is used to indicate the absence of the subject (they) from a location (at home).
آنها در خانه نیستند.
They are not at home.
In this case, 'نیستند' expresses that the specified items (the books) are not present at the stated location (on the table).
کتابهای من روی میز نیستند.
My books are not on the table.
The usage of 'نیستند' here is to emphasize that the subject (they) is not present at the specific event (the party).