brass vs high brass

brass and high brass both are nouns.

Word	Noun	Adjective	Verb	Adverb
brass	Yes	No	No	No
high brass	Yes	No	No	No

As nouns, high brass is a hyponym of brass; that is, high brass is a word with a more specific, narrower meaning than brass:

brass (noun)	high brass (noun)
a wind instrument that consists of a brass tube (usually of variable length) that is blown by means of a cup-shaped or funnel-shaped mouthpiece	brass with 35% zinc
a memorial made of brass
an ornament or utensil made of brass
impudent aggressiveness
the persons (or committees or departments etc.) who make up a body for the purpose of administering something
the section of a band or orchestra that plays brass instruments
an alloy of copper and zinc