Synecdoche is a figure of speech in which you use a part of something to stand for the whole thing. If your parents buy you a car and you say that you just got a new set of wheels, you're using synecdoche — you're using the wheels, which are part of a car, to refer to the whole car.
To correctly pronounce synecdoche, say "sih-NECK-duh-key." A synecdoche is a part that represents the whole. A photograph of a car that is completely covered in snow is a synecdoche for the burden everyone faces following a big winter storm. Synecdoche is a great literary device, especially for poets who strive to express a great deal in a single image.