If a company is unincorporated, it isn't formally or legally recognized. Your sidewalk lemonade stand, unlike Walmart or Costco, is unincorporated.
The adjective unincorporated can be used for something that's not fully combined or mixed into a whole, like unincorporated baking soda in your pancake batter that you need to stir a bit more. But it usually describes a business that hasn't legally registered with the government so that it can be considered a corporation. Unincorporated derives from the Latin incorporare, "unite into one body," with the prefix un- meaning "not."