Developers work in an alkaline solution. There are exceptions, but they these are uncommon. The chemical goodies that the developer works on are held in place on the film and/or paper by a binder of gelation. We are talking, salts of silver suspended in a coat of purified gelation called an emulsion.

Photo fluids are chemically dilute. In other words they are mainly water. When photo film and papers are submerged in the developer, the gelation emulsion absorbs the water, and like a dry sponge, it swells significantly. This action opens up the gelatin structure allowing the chemicals of the photo process to enter and percolate about and easily exit. The stop bath is a mild acid solution; in fact, is a solution of acetic acid (vinegar).

Films and papers are submerged in the stop bath after the developing step. This acid solution is quickly absorbed by the gelatin emulsion and, because developing takes place in an acid environment, all developing action ceases. Additionally, photo films and papers are still light sensitive at this stage, they need additional chemical treatment to render them light safe.

The third major chemical step of the process is a bath in a fixer solution. This solution is a solvent for residual light sensitive salts of silver. The fixer quickly purges the emulsion of light sensitive chemicals -- thus the film or paper is rendered permanent.
Next we wash or sometimes neutralize all residual chemicals of the process. These fluids, if not removed or chemically made inactive, will stain the finished image. The fixer is a sulfur based solution. If not removed, the sulfur will attack the developed image and stain it severely.

If you accidently place your film in the stop bath and follow this with a developer solution bath, likely the resulting film will be ruined or at best, the images will be substandard. Additionally, the stop solution will shift the developer solution towards the alkaline. This action will contaminate the developer so it must be discarded.

If you are aware of the mistake, stop bath first, likely you could save the film by washing. I would suggest a prolonged washing in running water for 15 or better 30 minutes. After this vigorous wash, you can proceed by restarting the developing steps from the beginning.