# Linear Predictive Coding - Meaning of spectrally Flat

I am a student with very little background on signal processing and am studying the Linear Predictive Coding algorithm. I came across the term spectrally flat in this article, https://ccrma.stanford.edu/~hskim08/lpc/ and am not sure what it means. I know that spectral is talking about frequency.

There are two possible signals for the source: an impulse train or random white noise. These signals model pitched sounds and plosive/fricatives respectively. The common characteristic for both impulse train and white noise is that they are spectrally flat; all spectral information is modeled in the filter.

So my question is:

1) what does spectrally flat mean ?

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