Skin bleaching is the process of chemically lightning over all, the natural color of your skin. This often involves powerful skin lightening agents such as hydroquinone or mercury.
Hydroquinone is available in the USA in concentrations under 2% over the counter and higher % by prescription at most med-spas and dermatologists) but has been banned all together in the EU for being very toxic to aquatic life, being suspected of causing genetic defects, skin cancers and may cause an allergic skin reaction or permanent skin discoloration.
Another commonly found and very dangerous "bleaching" agent is mercury, which has actually been used as a skin lightener since antiquity. Mercury is a highly toxic metal that can damage the nervous, digestive, and immune systems, as well as the lungs, kidneys, skin, and eyes. And is banned in both the USA and EU but can be found in bootlegged, illegally imported products .
Note that mercury may also be listed as mercurous chloride, calomel, mercuric, mercurio, or Hg in the ingredients lists. These products are found frequently on the internet on mass sale platforms like Amazon or even Etsy, and have been even found in some grocery/drug stores on ocassion.