It's not that complicated of an answer.
- developer to bring out the image
As I'm sure you know photographic film is coated in light-sensitive material (usually silver bromide). The thing is for black and white silver halide films, that the light does not react that intensely with the film, only a very few silver atoms get reduced. During development these reduced silver atoms catalyze and seed the reduction of surrounding silver ions to silver atoms by the developer chemicals (ussually hydroquinone). The more of these silver ions that are reduced, the darker the photo. Thus more light exposure gives more more initial silver atoms gives darker negative.
- "stop bath" to halt the process,
As you can imagine, if the development keeps going after enough time all of the silver will be reduced and the negative will just be black. The stop bath prevents this and stops development by using a dilute acid to stop the reaction of any residual developer chemicals which only react in basic conditions.
- "fixer" to make the image permanent.
Thus far the film has reduced silver to a desired point, but there is still light sensitive silver halide on the film. This silver halide is still light sensitive and over time with exposure to light will darken. The fixer bath uses sodium thiosulfate to make the silver halide soluble and dissolves the remaining silver halide from the film so that the image doesn't darken (it fixes the image in the sense that it can't change). At this point the film is no longer photo sensitive and can be view in daylight.