If you take a rectangle or a circle and stretch it out, you end up with an elongated shape called an oblong. If you stretch a circle until it becomes an oval, you've made it oblong.
First consider the Latin word longus, which means "long," and the prefix "ob," which refers to some direction. It makes sense, then, that an oblong should be an elongated shape, stretched long in one direction. It can be used a noun or an adjective. If your dining room table is an oblong, you're going to need an oblong tablecloth.