Black and white film photography has really interesting chemical processes behind it.
Processing and developing in photography is when film or paper is exposed to create an image. Processing film requires a controlled number for temperature and time. Film is sometimes soaked in water to swell the gelatin layer.
The developer then turns the image to particles of metallic silver. The Silver Halide crystals which were not exposed are light sensitive, so you need to flush them out of the emulsion. The stop bath, which is usually a diluted solution of acetic acid, stops the action of the developer.
Next, the image is dipped in fixer which makes the image not be light resistant. Rinsing the image in clean water removes the fixer. The fixer dissolves the crystals and makes them soluble which is then easy to wash out.