A
Darkroom chemical.
When you're finished with the entire chemical process necessary to make a photographic image and keep it intact, it is important to get it as clean as can be.
This can be done simply by washing the film/paper for a time period sufficient to get all the fixer off the surface. A quicker and better method is wash the film/paper briefly in Hypo Clear which converts sodium thiosulfate or ammonium thiosulfate into forms that are more readily soluable in water.