The word "床单" is formed by combining two characters: "床", which means "bed", and "单", which in this context means "sheet" or "covering". Together, they create a compound word that literally translates to "bed-sheet", directly describing the item as a sheet used for a bed. This formation reflects a common practice in Chinese where words are constructed by juxtaposing meaningful characters to convey a clear, functional concept, in this case specifying an object associated with sleeping furniture.