Builder Enabled (the checkbox labeled "Builder Enabled (use builder output for this page)" in the Builder section of the form editor) decides which layout visitors see: off, the form on your site renders from the classic field list; on (and saved with the Save Builder button), the form renders the custom HTML/CSS you built on the canvas instead.

Important: clicking Save Builder re-syncs your fields to match exactly what is on the canvas – any field that exists in the classic list but was not dragged onto the builder canvas has its field definition permanently removed. Answers already collected for that field aren't explicitly wiped, but because the field itself is gone they become orphaned and no longer appear anywhere in your results – for practical purposes, that data is lost to you. (This is different from deleting a single field the normal way in the field list, which does explicitly clear its collected answers too.) Fields you did drop on the canvas keep their settings and stored answers. So once you start using the Builder for a form, add and remove fields from the canvas rather than from the plain field list to avoid losing data.