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.