The metric is a measure of how much of the frequency spectrum a particular modulation format takes up. For example traditional OOK (on-off-keying) has a spectral efficiency of 1bit/s/Hz. In coherent optical communications we take ideas from radio to send more bits in the same amount of BW by encoding information in the phase and amplitude. For QPSK, we send 2 bits/s/Hz. In optics we use 2 orthogonal polarizations at the same time so we call this DP-QPSK. This doubles the spectral efficiency to 4bits/s/Hz.

One can keep going to higher order modulations to improve the spectral efficiency but the SNR required increases exponentially with constellation order.