The phrase 'polyphonic ringtone' is used more and more in todays mobile phone/cell phone jargon. Basically it's a fancy way of saying that the phone can play multiple sounds at once - producing a more pleasing sound. First generation phones contained a 'buzzer' which was only designed to produce two types of beeps (using different currents would produce different tones of beeps). The polyphonic design is really a cheap option for mobile phone manufacturers because it produces good sound quality without implementing more expensive circuitry to reproduce digital sound (DSP) like the sound card in your computer.

Polyphonic ringtones are generally MIDI files converted to 'scalable polyphonic MIDI' files (they're optimised for the standard 4 channels supported by the phone). They can be uploaded to your phone via a data cable or infrared (IR) port or purchased on the web which sends it straight to your phone. See for an example of ringtones, pictures, etc.