I’m assuming you’re not super familiar with PCB fabrication? The toner transfer is a way to get small, repeatable, and precise definition of the component pads and traces - at home - without needing the expensive industrial machinery that fab houses have.

The toner ’print’ is a negative image, placed on a sheet of copper mounted to (usually) fiberglass. Much like masking off areas when painting, the ‘print’ protects the copper surface in select areas you want to keep safe when the whole board goes into the acid bath to dissolve the unprotected copper - leaving copper only where your ‘print’ was, and hopefully no shorts/grounds.

Definitely cool to have the ability to DIY fine pitch if you’re manually mounting SMT components or want higher board density, beats out the hand drawn permanent marker lines I’ve done in the past 😅