what's the differences between dB/Hz and Hz in the Power Spectral Density.

Dear colleagues,

I'm comfused with a smaller issue with Power Spectral Density. In many papers, scientists always use dB in the y axis lable (see Fig.1).

 But the y lable is dB/Hz (see Fig.2) in the examples of PSD documentation. plz give me some hints how to realize their differences! Thank you very much!| |