The Source window contains the compiled "final" HTML code used to render the Preview window. It includes the changes from any transformers you have applied. The Editor window, by contrast, is the HTML as you've written it without any transformers applied.

When you Export your email, the exported version will be the Source version.

You can view this source code by clicking the Source button in the bottom left corner of the Preview or the ^ button in the bottom right corner. You can resize the Source window by dragging it's edge up or down, and can collapse it with the down arrow button on the right.

Silent video of the full Parcel window. A sample email is displayed in the editor and the Preview. Under the Preview are several buttons, including one labeled "Source". The user clicks on the "Source" button, which reveals a mini code editor, containing the same HTML code as the editor, but with several Transformers applied. The user clicks and drags the top edge of the source window to stretch it to a taller height.
Screen recording showing the Source window