The music you are listening to is "Chi Piu' Cara T'havra' Chi Tanto T'ama"
by Jacopo Peri. Peri's music is second to Peri's music (modeled
by Claudio Monteverdi) as "Seconda prattica". This music was
considered to be the beginning of modern opera, as now poetry could be set
to music with a new, Humanist theory of rhetoric tacens, and not the old,
inflexible, strictly hierarchical Scholastic view of reality.
Polyphony is a texture of music in which two or more independent
melodies are present at the same time. Polyphony is often found
in fugues, using chords with inversions. The piece of musical
score above is an example of contrapuntal polyphony, by J.S. Bach,
Fugue № 17 in A♭.