Metonymy is a speech figure in which a thing or concept is not mentioned by the actual first name, but by the name of an associated thing or concept. Or, to put it differently, one element of a larger concept becomes the symbol for the entire concept.
For example, Wall Street is both a literal place (a street in New York City where important elements of the American financial sector) market are located such as the New York Stock Exchange) and a term used to represent the American financial market . When a newsreader says "Wall Street has been shaken today", they use metonymy because they do not refer to the literal street, but the entire financial sector.
Other well-known examples of the phenomenon are Hollywood (it is literally a district of Los Angeles, but the word serves as a catch-all for the entire American film industry), Washington (literally Washington, DC, but used as a stand-in for the US government) and Scotland Yard (the old headquarters of the London Metropolitan Police Force was on Great Scotland Yard street and now the street name stands for the real name of the office.
Metonymy is also not limited to We call porcelain / ceramic bowls "china" because of the historical association between the product and China, refer to mercenaries as "rental weapons" (in reference to their tools), refer to military officers as "brass" (in reference to the metal) alloy traditionally used for military buttons and insignia), etc. Once you start looking for it, metonymy is all around us.