Voices provide a way to subdivide an instrument into its own separate parts, which play simultaneously. This can be useful for polyphonic instruments, that is, instruments that can play more than one note at a time, e.g. guitar, piano.
piano:
V1: c d e f g1
V2: e f g a b1
V3: g a b > c d1
V0: c4 e g > c2.
Each voice is its own sequence of note events. The first note/rest
in each voice starts at the same time, like the notes in a chord.
Whereas a chord bumps forward the current offset by the shortest
note duration in the chord, after a group of voices, the current offset is that
of the longest voice in the group. V0: signals the end of a voice grouping and
a return to using a single voice -- the first note placed after V0: will
happen after all voices in the group have finished.