Questionnaire Marketing Source
At the top of your questions page, you'll notice a dropdown with the label "Track your marketing sources". If you haven't created any additional questions yet, the currently selected (and only) option will be the default question, "Where did you hear about us?"
As you may have determined, the question you select from the list in the dropdown will serve the special purpose of tracking where your contacts are hearing about you - data that is used in materials like Spark's Source Report.
You are not required to use the default question, although you can edit the text and possible answers to suit your needs. Any question you select will have some restrictions applied to it automatically - It will be marked as a required, single-answer question on your forms, and while it is selected as your source-tracking question, it cannot be deleted.