A goose is a large bird with webbed feet. Geese hang out around ponds and lakes, fly in a V formation, and make a distinct honking noise.
Geese are classified as waterfowl, birds that live at least part of the time in a body of water. If someone calls you a goose, it's mildly insulting: they're saying you're silly. Many other common phrases use goose, including "a wild goose chase," which means a hopeless pursuit or waste of time, and "your goose is cooked," which translates to "you are in big trouble!"