An ermine is a wild animal, a small mammal with a very soft winter coat. Ermines look a bit like large weasels, unless you are wearing them as outerwear.
You're likely to come across the noun ermine in connection with the way humans use them — in some areas of the world, they are commonly raised or hunted for their fur, which is prized for its softness and can be made into coats or hats. Another word for an ermine is a stoat. A coat made from an ermine's skin is also called an ermine.