Question banks are containers for question slides. You put questions in, then draw them out when you need them. Reuse questions as often as you'd like throughout your course.
Question draws can include all the slides from a bank or just a subset, and you can randomize the questions to keep quizzes from being predictable. To learn how to manage question banks and question draws, see these tutorials:
Creating and Managing Question Banks
Adding and Editing Slides in a Question Bank
Drawing Slides from a Question Bank