What exactly does C/No (dBHz) mean in u-Blox GPS data?
Looks like a form of Noise spectral density. In fact there's another Wikipedia article for this: Carrier-to-noise density ratio


  C/No is the ratio of carrier power to the noise power mixed with the signal, in a 1Hz bandwidth. 


Basically, it's the ratio between the amount of signal power (practically measured as the carrier power with the obvious unit of watts) and the amount of noise power density (unit of watts per hertz) you receive into the receiver. Since it's in a 1Hz bandwidth, you can express the unit as dBHz (dB is the quasi unit for ratios of powers, per hertz of measured bandwidth)

This is just a figure to indicate how noisy the incoming signal is.