Literally, many sounds. In Music it refers to the sounding of different notes simultaneously as in different melodic vopices.
It also refers to the ability of instruments to sound more than one note at the same time. Stringed Instruments (examples: Guitar, Mandolin, Piano, Dulcimer even Fiddle) and some Reed Instruments (examples: Concertina, Melodeon, Accordian, Harmonica ) have this ability while simple wind instruments (examples: Flute, Bagpipes with single chanter, Penny Whistle) are Monophonic.
The term is often used of electronic keyboards to describe how many notes can be played together.