What is it?
Images which have been developed and/or printed using film and/or photographic paper containing light-sensitive chemicals, specifically silver halides, by way of traditional darkroom techniques.
Trad process submissions are, simply, film shots or prints from a darkroom that aren't using weird experimental/historical processes or instant media. The trad process gallery is one of the technique-based galleries, which means that every submission would fit equally well in one of the theme-based categories like APN or Conceptual depending on the subject of the photo; by submitting to trad processes you're inviting criticism of not only the shot itself but also the medium you chose to capture it with and the choices you made in processing the final image you've presented.