Field mapping is how you tell DayBack which attributes of your events should show up in the calendar.
There is no field mapping required in Google. Office, or Basecamp calendars since DayBack already knows what those events look like. Though you can create custom fields in some of those sources.
But for data sources when you get to create your own tables or objects--like in Salesforce or FileMaker--you'll want to tell DayBack which of your tables should be treated as calendars, and which fields to use in each table.