This field will be positioned at the bottom of your ballot. You can collect comments or any text field data with the text box. The default field name on the ballot is "Ballot Comment".
You can however change the field name to anything you like. Goto Configuration > Wording on the sidebar, then scroll down to Ballot Comment. In the Ballot Comment field, enter the new field name. For example you could change the name to "Enter Email Address". Once complete, scroll down to the bottom of the page and click the Save Wording button to save the change.
Note: The comment field isn't required to be filled-in to submit a ballot.