If it is a verb, like "I spotted a error in his homework.", it means you noticed something that is hard to find(?), or the thing you are searching for. Or it could means that you saw someone you are searching for, like "I spotted xx in the crowd." If it is a noun, it means that something have spots on it, like "a cat with spotted skin." Hope it helps. :D

A leopard's skin is spotted, full of spots.

If you say 'I spotted John in the crowd of people' it means I saw him.