The "Edit Source" button under the "Advanced" tab in the toolbox opens the "Edit Source" dialog.
The "Edit Source" icon in the top right corner of the form area opens the "Edit Source" dialog.
This is an advanced feature and it must be used very carefully.
Form Builder uses XHTML+XForms as form definition representation. The format must be well-formed XML. The dialog shows a text editor containing that representation.
It is possible to edit the XML representation in the dialog, or to copy/paste it to an external editor for further changes. After saving changes from the dialog, those are immediately taken into account by Form Builder.
NOTE: You have to be cautious when doing so because it is possible to break the form by making changes! We recommend that you learn about XForms before making such changes.