yes,getByteFrequencyData results in a normalized array of values between 0 and 255.
(it copies the data to the array it gets passed-in).
the frequency bands are split equally, so each element N of your array corresponds to:
N * samplerate/fftSize
so, the first bin is 0.
and, assuming a samplerate of 44100 and a <analyzerNode>.fftSize of 512 the second would be: 86.13 Hz, and so on…
you will find these two questions and answers useful, on dsp and on SO:
Note that the length of your sampledata is half the <analyzerNode>.fftSize, effectively limiting the frequency-range to half the samplerate.