Brass is a shiny, gold-colored metal that is an alloy of zinc and copper. You'll see brass everywhere once you start looking: in doorknobs, lamps and light fixtures, zippers, and tools.
Because brass is both workable and strong, it's used in musical instruments (in fact, there's a whole category of instruments known as brass, including saxophones and trumpets). It's also a low-friction metal, so it's preferred for ammunition casings, gears, and locks. And brass is shiny and cheerful, so people like to use it in light fittings, plaques, and sculptures. High-ranking military members are informally known as brass, inspired by the insignia on their uniforms.