Audi is a German car brand. The same big company group also makes Volkswagen cars, so you may hear the two names together. Audi cars are often seen as a bit smarter or more expensive than a plain family Volkswagen, though that is a feeling people have, not a rule. You can say 'an Audi' for one of the cars. It is a proper name, so it takes a capital A. In Taiwan you see the four rings on the road and in ads; people treat the name as a brand, the way they treat Toyota or BMW. Though actually, Audi is only the brand name here — it is not a general English word for 'car', and it is not the English word audio.