A lot of things happen when you immerse your film in a developer. Since the developer is primarily water, it wets the film. The emulsion consists of light sensitive salts of silver. These are imbedded in a binder (glue) made of gelatin. The water causes the gelation to swell. This action opens up the gelation structure which resembles a jumble of transparent spaghetti. The silver salts are imbedded between the strands.
Now that the structure is swollen, the developer is free to percolate within. The developer seeks only exposed salts of silver, and when these are met, it reduces them to metallic silver by liberating the halogen component (bromine – iodine – chlorine). The liberated halogens are dissolved into the water of the developer.
All this swelling, infusion and reaction take time. Fast films (higher ISO) contain more silver salts -- thus the process takes longer. Slow films with lowered ISO, develop more rapidly. Developer manufacturers test their solutions on various film emulsions and publish their findings in the form of a Time-Temperature table. The developing time is different for high speed films, because they contain much more of the light sensitive goodies.
We can push-process: This is adding developing time to force the film to perform with an ISO that is greater than intended. We can pull-process: This is reducing developing time to correct for an unintentionally over-exposed film.
Additionally we extend developing time to achieve results that upraise the natural contrast of the film (yield a harsher image). Conversely we reduce developing time to reduce contrast (flat image).