Why input signal level is measured in dBm/Hz?

I saw a datasheet indicated the input signal level is -173 dBm/Hz.

Why dBm/Hz ? Could someone please kindly advise and thanks.

In case your input signal bandwidth is 1 Hz, then the input sensitivity level is -173 dBm. That is about 1 dB higher than the thermal noise floor in room temperature. BUT, you might have wider input signal, let's say 10 kHz. then you should apply this signal to your system @ -133 dBm level...

Got that?