What does it mean that the normalized power of a received equals one?
In mundane words: the concept is very close to that of orthogonality. For two components of $x$, say  $x_i$ and  $x_j$, if they are different ($i \neq j)$, the average outcome of their product is zero (or $E(x_i x_j^H)  =0$). If   $i = j$, one assumes that the energy is normalized to one (or $E(x_i x_i^H)  =1$). 

Resultingly, the product matrix is a unit matrix: one for the diagonal ($i = j$), and zero outside.