If you have a lot of polls they can pile up over time. In these cases you might want to take one of two options:
If you archive a Poll it will no longer be visible in your dashboard. You will still, however, retain all of the data associated with this poll. You will also be able to unarchive the poll at any time.
You should only delete a poll if you are 100% sure you no longer want it or any of the data associated with it. It is usually a better idea to archive a poll, but if you are certain you want it gone, this option is available to you.