A closed question is one that has a yes or no answer or a simple, fact-based answer. Such questions lend themselves to the kind of short answers that a person having difficulty understanding another person might be able to comprehend.
Examples of closed questions would be to ask the person what city and state they were born in or how old they are. They could also be the kind of question that might be answered on a questionnaire, such as "Do you own a home?" or "Are you a high school graduate?" These both have a yes or no response.
An open-ended question, in contrast, is one that allows for the responding party to offer opinions, thoughts, and reflections. It invites much longer responses.
While closed questions in the context described would make it easier on the hearing questioner, they have the disadvantage of limiting what the deaf (or Deaf) person is able to communicate, and the questioner might want to find another form of communication that would allow the respondent more freedom. Many would find something inherently uncomfortable and dehumanizing about limiting the ability of a person with a disability to communicate.