Or: the mad
smelly stuff in your
Darkroom.
Fixer is one of the key photographic chemicals, nearly always almost straight sodium thiosulfate or ammonium thiosulfate.
Fixer's sole purpose is to make light sensitive photographic materials (negatives and photographic paper) insensitive to light. It does this by removing (dissolving) all the unexposed silver halide from the material while leaving the developed silverintact.
Putting the film/paper through fixer is one of the last stages of development, and until processing the material through the fixer it is imperative to keep the material away from light. Those who ignore this warning seek ruin.
Hypo (after
Sodium hyposulfate, the chemical later renamed as
sodium thiosulfate) is another word interchangeable for
fixer, but it gets less usage these days than it once did. Oddly, the chemical you use to make sure all the
fixer is cleaned away from the material is still referred to as
Hypo Clear.
Go figure.