Polling is the ability to receive touch-tone responses from call recipients during a voice broadcast campaign. It is an extremely convenient way to gather simple responses from a large group of people. (Although not generally described as "polling", you can also receive responses when you send a text blast.)
For example, polling could be used to gather volunteers for an event: "This is John with XYZ non-profit group. We need volunteers at our event on Saturday evening. If you are interested, please press 1, otherwise press 2. Thank you!"