Perrier is a famous French sparkling mineral water. It comes from a spring at Vergèze, in the south of France, and the bubbles are natural — they are already in the water, not pumped in later the way they are with many cheap fizzy drinks. People order "a Perrier" the way they order a brand, and sometimes, a little loosely, they use the name for any posh bubbly mineral water, the way some people say Kleenex for any tissue. In a restaurant it often just means "the sparkling one, the French one," with a slightly expensive flavour. Write it with a capital P: it is a brand, not a generic English noun.