Either a small furry creature who lives underground and digs up people’s lawns, or a unit of quantity in chemistry a bit like a dozen. A mole in chemistry is 6.022 x 10 to the power of 23 atoms or molecules. That’s quite a lot.
A mole could also refer to a skin lesion which may be darkly coloured. The dark colour is because there is a high concentration of skin cells known as melanocytes which produce the dark pigment, melanin. Some of us are born with moles while other moles are acquired as we get older. There are many different kinds of moles, depending on their shape and colour. You can read more about them here http://www.webmd.com/skin-problems-and-treatments/guide/moles-freckles-skin-tags
Most moles are non cancerous. However, some moles can become cancerous. Hence, it is important to see a doctor if you notice changes in the appearance of existing moles.