Noise is not deterministic it has a variance and other properties of a random type.

dBc means ratio in amplitude between the carrier and some point

here we have some random thing so how ding /HZ helps us catch soe value from it?

https://en.wikipedia.org/wiki/Phase_noise

Phase noise (ℒ(

*f*)) is typically expressed in units of dBc/Hz, and it represents the noise power relative to the carrier contained in a 1 Hz bandwidth centered at a certain offsets from the carrier. For example, a certain signal may have a phase noise of −80 dBc/Hz at an offset of 10 kHz and −95 dBc/Hz at an offset of 100 kHz. Phase noise can be measured and expressed as single-sideband or double-sideband values, but as noted earlier, the IEEE has adopted the definition as one-half of the double-sideband PSD.