In classical music, a mezzo-soprano is the second-highest singing voice. While they don't sing the very highest notes, mezzo-sopranos reach higher registers than contraltos.
In Italian, mezzo means "middle," and soprano, "the treble in music," is from a root meaning "above." Mezzo-sopranos are right in the middle, between soprano and contralto voices. Classical music lovers describe mezzo-sopranos as sounding darker and richer than the brighter, higher tone of a soprano. If you want be an opera or choral singer but you can't quite reach those high notes, you may be a mezzo-soprano!