What is exactly Power Spectrum Density?
Power Spectrum Density (PSD) represents the spectral distribution of total power of a given WSS random signal. Conventionally, it's defined as the Fourier transform of the Auto-Correlation function (sequence) of the WSS random process as:

$$ P_x(\omega) = \mathcal{ F } \{ r_{xx}[m] \} $$

where the auto-correlation sequence (ACS) $r_{xx}[m]$ of the WSS $x[n]$ is defined by:

$$ r_{xx}[m] = E\{ x[n]x^*[n-m] \} $$

These are theoretical entities, you may not compute them in practice where only a single or a few realizations of the random process are available. In such a case, the PSD is estimated and there are different estimation techniques, one of the most common and simplest is known as the periodogram estimate of PSD and given by

$$ \hat{P}_{x}(\omega) = \frac{1}{N} |X(\omega)|^2 $$ 

where $X(\omega)$ is the discrete-time Fourier transform of the observed signal $x[n]$ of length $N$.