The program uses a microphone to analyze sounds in real time, and it displays what it hears on a scrolling graph. Pitch is indicated by the height of the trace, loudness is shown by the thickness of the trace, and the color indicates intonation relative to a set of reference pitches -- red for sharp and blue for flat.
Although it was originally developed strictly for studying intonation, it turns out to have other uses as well. It's a darn good tape recorder for any purpose, including recording yourself and playing it back to see how you sound. If it's a solo instrument you're recording, the visual pitches make it easy to select a passage for playback. A lot stuff that takes careful listening to hear jumps out at you in the display. It also makes a decent transcription tool.