How to derive pitch from spectrum data?

I'm trying to write a program which derives pitch from a monophonic audio source - a single instrument. I found this method in AudioSource called GetSpectrumData. However, it isn't very clear what spectrum data actually is. Is it audio data after an fft or before?

Also, what data is it actually putting out, is it a representation of a range of frequency amplitudes over the sample range. Or is it simply giving the overall amplitude of each sample?

Has anyone had experience with this? Do you know what spectrum data actually represents and how I might derive pitch from it?

Documentation at: http://docs.unity3d.com/Documentation/ScriptReference/AudioSource.GetSpectrumData.html

Thanks! You guys are great.