The chemical pigment found in the skin & organs of plants & animals. In humans it is what makes our skin dark ot light. If you high levels of melanin you have dark skin and if you have low levels you have light skin. You get tanned when you sit out in the sun as your skin protects it's self to make more melanin. Melanin is also used by the body to colour hair and eyes.
If you have no melanin or can't produce it, this is known as Albinism.
If you have excessive amounts of melanin it is known as Melanism