This poem is using terminology common in developing photographs by hand in a darkroom. "It didn’t always work this way." (Referring to how photos aren't often processed by hand anymore) "There was a time you had to get your hands dirty, when you were in the dark for most of it. Fumbling was a given." (Talking about working in a darkroom, fumbling because it is hard to see) "When you needed more contrast, more saturation- darker darks and brighter brights- they called it extended development." (When processing photographs by hand, you literally control the development of the film. Depending on what you do with the chemicals and exposure with light, you control the contrast and saturation. Darker darks and lighter lights is a common phrase in photography and other arts, where you want the darkest parts and lightest parts of an image to be very bold.) "It meant you spent longer time up to your wrists, longer time inhaling chemicals. It wasn’t always easy." (Literally talking about working with the chemical baths that the film soaks in, and all the hard work involved with developing photographs.) I hope this helps! Let me know if you have any other questions. :)