What copper is and what it is used for in horse's feed - I'm talking about the copper powder which you can add to horse feeds.
Good idea for a thread, but one that I think is too broad for one thread.
But to answer your question, copper is a 'trace mineral' meaning that it's an important, necessary nutrient but needed in tiny amounts. It is mostly needed for hemoglobin, red blood cells & bone development, but is also important for production of collagen & elastin(so an important 'ingredient' of healthy hooves, skin, coat, and healing of wounds). It is also important for skin/hair pigmentation & it is said that black horses need more than lighter colours, and that feeding extra copper to a horse will prevent sun bleaching. Adequate copper levels also helps reduce susceptibility to infections, especially fungal.
Nutrients need to be balanced appropriately with many others, and copper is particularly strongly linked with zinc, iron, calcium levels. It is also a heavy metal(which means it doesn't get excreted, but accumulates in the liver) and excess amounts are very toxic.