Questions are pre-built snippets of HTML that you can use on your actions to store data in structured ways in your custom fields. (Questions can also tag activists, see here for more about tags.)
For example, you might collect phone numbers from your activists on all of your action pages. You can make a question that displays a text box with a label that says "Phone Number" and stores entries in the "phone_number" custom field you're using for this data. Once you create the question you'll have able to drag it in to any action form without having to create it from scratch each time.
Generally, creating a question will create the corresponding custom field if it doesn't exist already.
Note: Activists may only have one value per custom field on their record. So if they enter data in a question again, it will overwrite any old data in that custom field.