How to understand GetSpectrumData?

Code (JavaScript):

1. var spectrum :float[]=AudioListener.GetSpectrumData(1024,0,FFTWindow.Hamming);
2.  
3.  
4. var c1:float=spectrum[1]+spectrum[1]+spectrum[1];
5. var c2:float=spectrum[3]+spectrum[3]+spectrum[3];

Hello,Could sombody explain what it above means? How to read it?
if i get it right there is 0-1024 units of something which reprezents 0-22000Hz
And now: if 22000Hz / 1024 then spectrum[1] = around 21Hz Frequency of sound

so------var c1 has 1+1+1=3 then it is around 3 x 21=63Hz
1.but what is exactly that "c1" is it a number from 0-63Hz taken when such freq is heard by lisner ? or maybe something else

2.what if for exp somebody wants to get numbers between 10 000hz-22000hz
3. Why var c1:float=spectrum[1]+spectrum[1]+spectrum[1]; is not =var c1:float=spectrum[3]
and why is it in parts?

 

the spectrum array repesents powers of various frequency bands
spectrum[1] is the power of a certain band
spectrum[2] is the power of a higher band, spectrum[3] for a further higher band, etc
each band covers a multiple of the fundamental frequency of the interval used.
spectrum[1] +spectrum[1] +spectrum[1] is just 3 times the power of band 1 (whoever wrote this example was dumb, i've seen it before) you can just write spectrum[1]*3

 

so GetSpectrumData is like a cake of freq sliced in pieces (in this exp 1024) and evrey piece holds some range of frequencys? but these slices are not equly big
is it mean that?
spectrum[10] =10*10=100; since 1=around 21hz then 100units is around= 2100Hz then spectrum data takes the value of avarage amplituede of the slice that this 2100Hz is within >and as music is changing so the amplituedes of that 2100Hz.. Is it more correct? or still i didt get it