A closed question, or closed ended question is one that can be properly and sensibly answered with a straight yes or no response OR a specific short factual answer. With closed questions, the idea is that there is one correct answer.
An open ended question is one that cannot be answered with a simple yes or no question, and often has no single right answer.
Some trainers and instructors view closed questions as "bad", and open ended ones as good, but that is false. Each has its uses.
Closed questions, because they stimulate short yes/no answers are ideal for creating a fast pace interaction between instructor and group members. Used in a machine gun fashion they can increase group energy, and give a sense of urgency to the process. However, they should not be used in this fashion for too long since the power to energize is based on the questioning sequence being unusual or novel to the group, and short lasting.
Clearly, closed questions are not ideal to stimulate in depth thinking, and that's the major function of open-ended questions.
Open ended questions should be used to generate longer, non-superficial discussions, promote critical thinking and otherwise create intellectual involvement.