The Load Default Fetch option will automatically load a fetch based on the current CRM Entity. Typically, this will include the unique identifier field for the chosen entity, as well as the name field for the respective entity. For example, if you were to use the Accounts entity, the default fetch would include the account id and account name. This would appear like this:
Please note that the Value and Display text boxes inherit their contents from the attribute names in the FetchXML box below. Remember that the Display text box will be what options are visible for you to select while the Value text box will be what will actually gets selected and submitted through the form.
The FetchXML also includes an advanced option for choosing exactly what fields you would like to be retrieved by the widget. When you click on the Build option, you will be directed to a page like this: