tone poem vs piece of music

tone poem and piece of music both are nouns.

Word	Noun	Adjective	Verb	Adverb
tone poem	Yes	No	No	No
piece of music	Yes	No	No	No

As nouns, piece of music is a hypernym of tone poem; that is, piece of music is a word with a broader meaning than tone poem:

Other hypernyms of tone poem include composition, musical composition, opus, piece.

tone poem (noun)	piece of music (noun)
an orchestral composition based on literature or folk tales	a musical work that has been created