The Autocomplete option in Short Text, Long Text, and Email gadgets allows you to configure an autocomplete suggestion for a form field.  By utilizing the user's browser autocomplete settings it will suggest a value on a form field for selection which can be used to improve the built-in accessibility of your forms. 

Autocomplete configuration will be omitted by default (even if autocomplete is not enabled, the browser still could offer hints).  If you don't want to include autocomplete at all you'll want to disable via the 'Off' option.  Below outlines the options when autocomplete is enabled on the field:

• On (Browser Default) - the default autocomplete logic set up in your browser settings will be used for possible suggestions.
• Off - the browser will not provide any autocomplete suggestions for the field.
• Name - will suggest the Name configured in your browser autocomplete settings.
• Phone number - will suggest the Phone configured in your browser autocomplete settings.
• Street Address - will suggest the Street Address configured in your browser autocomplete settings.
• Postal Code - will suggest the Postal or Zip code configured in your browser autocomplete settings.
• Custom - allows you to configure a custom autocomplete option via a html attribute.  More information in the link on how to set that up (requires a bit more technical knowledge).

More information can be found on this new setting in the Gadget Configuration Settings article.