We all know of the human voice as an instrument. In keyboard and MIDI terminology a voice is one of many possible notes/sounds that can be played simultaneously on a synthesizer or sample based electronic instrument. A monophonic keyboard can only play 1 note at a time, and is therefore considered a 1-voice instrument. A 6-voice instrument can play 6 notes at a time, while a 128-voice instrument can sound 128 notes at once. This is also known as polyphony (see Polyphonic). A 64-voice instrument can be said to have 64 notes of polyphony, which means you can get a maximum of 64 notes (or events) happening at one time. It has 64 voices.
Occasionally you will see the term voice used to describe what would more accurately be termed a timbre or patch in a keyboard. If a keyboard can make 10 different sounds there are some who will say it has 10 voices. This is confusing and most professionals try to avoid the use of the word voice in that context, favoring terms like patch, timbre, part, sound, program, or any number of other terms.