As I understand it, interragtories are basically detailed questions that must be answered by a debtor to the court re: things like assets which might be used to satisfy a judgement.
For example, if a court ruled against your sister over a defaulted credit card bill and a judgement was entered in favor of, say citibank, Citibank could then file with the court a detailed list of questions, such as list all of the bank accounts, numbers, amounts etc... provide wage stubs, tax returns etc... basically any and all things which could help a court determine a debtors financial standing and ability to satisfy the judgement.
Others may be able to add more, but that's what I think it is in a nutshell.
One quick point, I cut and pasted your spelling of the word and it is spelled "interrogatories"
Here's a link with some additional info
Interrogatories - Wikipedia, the free encyclopedia