What can we expect when receiving a letter for SSI that says we need to talk to the person for the claim regarding SSI beneifits?

We received a letter today from social security that says that they want us to bring in my father (who we filed the claim for) to the social security office because they need more information before getting SSI benefits. The letter goes on to say that he has been approved for medical disability. What will they ask or be looking forward in this meeting? My dad is in a rehab facility and has mental problems in response to his disability. I am nervous for what they will ask him. Anyone have any experience with this?