Short for the brass family, brass refers to the trumpet/cornet, horn, trombone, euphonium/baritone, and tuba sections of the band. Nearly all of the main characters of Hibike! Euphonium play brass instruments.
Many people wrongly assume that the saxophone is also a brass instrument because of its color, but the saxophone's use of a single reed mouthpiece makes it a woodwind instrument.
A list of brass players in Hibike! Euphonium can be found here.