What is a Requirement Document?
It is a document that describes the details of the application functionalities which is required by the user. In other words, this documents tells stakeholders (developers, testers, Project Managers, Business Analysts and other related people) what functionalities are going to be developed in the application.
Note: For the interview purpose, I would recommend that you don’t try to read this document in detail. Just have an idea. But you should be able tell thoroughly the above line of definition.
I have attached a link to the requirement document just to glace how it looks. This document is a courtesy of softwaretestingclass.com.