A galosh is a shoe you can wear on a rainy day. Galoshes are usually made of rubber, and sometimes slide on over your regular shoes.
A galosh is one kind of wet-weather footwear, most often made from a stretchy kind of rubber so it can be slipped on to protect your shoes from getting damp. You're most likely to come across this word in its plural form, galoshes, since they come in a pair. Galosh comes from the Middle English, for a sort of clog, from gallica solea, "a Gallic sandal" in Latin.