There are chemicals that are photosensitive - that undergo chemical reactions when exposed to light. The energy from light hitting the chemical causes it to either become unstable or facilitates a reaction with surrounding chemistry.

One example is silver nitrate - when exposed to light it decomposes and one of the products is a black/brown substance. This reaction stops when the light source is removed. So you can expose a film/glass plate coated in silver nitrate to light / the parts that get a lot of light turn black, the parts that get no light stay clear and everything else is somewhere in between. When you stop the exposure the reaction stops.

You then rinse the film/plate with another chemical that either washes away or reacts with the film chemical to form a stable compound that is ideally clear to “save” the film. Now you can take it out and expose to light without it degrading.

This is a negative though - the light parts of the scene are dark and vice versa. So you shine a light through it onto photo paper with similar chemicals to reverse the negative and then use other chemicals to cause a reaction on the photo paper and then stop it to get the finished photo.

This is also why it took a while to get color photography - for this you need three sets of compatible chemicals - three photosensitive chemicals for the film that react with specific wavelengths, a chemical that will lock the exposed film, and then three more chemicals for the photo paper that react with the specific wavelengths that come through the negative and produce colored compounds that correspond closely to the original colors of the scene that was photographed, and then compatible chemicals to wash/fix the finished photo without damaging the one or more of the other chemicals used.