Linear Predictive Coding - Meaning of spectrally Flat
1) what does spectrally flat mean ?


Spectrally flat means that the signal has energy at most / all frequencies, and this energy is roughly the same level across the frequency range.


  2) why does an impulse train have a flat spectral ?


As you're probably aware, the Fourier transform of an impulse train is also an impulse train. That means that the claim by the link that the impulse train is spectrally flat is not correct.

The reason an impulse train is used in modeling speech is because it gives a tonal quality for non-fricative sounds (mostly vowel sounds). That tonal quality is hard to generate by filtering if the source being filtered is white noise.