Bleaching is a technique used to lighten certain areas or the whole of the print, not to be confused with household bleach! but a mixture of potassium ferricyanide and fixer.
If used at to strong a dilution it will bleach the whole print back to the white paper base, but used carefully it can be used creatively to lift cloud scenes, highlights or shadow detail. My personal preference is to always lightly bleach the whole print very briefly, it just lifts that extra little bit of detail out of the print. Any small areas that need work I use a small brush or cotton wool bud and have a wet sponge on hand to arrest the bleach action when needed. After bleaching the print needs to be fully washed to remove all traces of fixer.