From Halopedia, the Halo wiki
Brass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. In comparison, bronze is principally an alloy of copper and tin. Despite this distinction, some types of brasses are called bronzes. Brass is a substitutional alloy. It is used for decoration for its bright gold-like appearance; for applications where low friction is required such as locks, gears, bearings, ammunition, and valves; for plumbing and electrical applications; and extensively in musical instruments such as horns and bells for its acoustic properties. Brass has a muted yellow color, somewhat similar to gold. It is relatively resistant to tarnishing, and is often used as decoration and for coins. In antiquity, polished brass was often used as a mirror. Brass has likely been known to humans since prehistoric times, even before zinc itself was discovered. It was produced by melting copper together with calamine, a zinc ore.
Brass was still being used as a shell casing for firearms in 2552. Ash put spent brass casing on the graves of William and Dante. It was also used for lamps. One of these lamps was found on Vice Admiral Ysionris Jeromi's Koa Wood desk in 2551.
In Halo, as well as present day military, senior commissioned officers are often referred to as "the Brass". The nickname was probably bestowed because their insignias are/were once made of brass.