De Wikipedia :
The English compound noun cellar door is commonly used as an example of a word or phrase which is beautiful in terms of phonaesthetics (i.e., sound) with no regard for semantics (i.e., meaning). It has been variously presented either as merely one beautiful instance of many, or as the most beautiful in the English language; as the author's personal choice, that of an eminent scholar's, or of a foreigner who does not speak the language.
The semantics of "cellar door" derive straightforwardly from its component terms: in the United States, a cellar door is often a door or pair of shutter doors between the outside of a building and its cellar. In Britain, Ireland and Canada, a cellar door is often located within a house and opens onto a flight of stairs leading to the cellar. Outside doors are more common to pubs and restaurants.
Nunberg further suggests the semantics of "cellar door" are not actually irrelevant; in fantasy, a mundane door can become a portal to another world, as with the wardrobe of The Lion, the Witch and the Wardrobe or the rabbit hole of Alice in Wonderland. This idea is utilized in the 2001 film Donnie Darko, where the phrase "cellar door" is discussed in one scene, and an actual cellar door figures into the plot in a later scene.