It is ratio of the wanted signal (Carrier) to the noise floor (Noise). The higher the C/N ratio, the easier the baseband chip can demodulate the signal without errors. More complex modulation constellations require higher C/N to successfully demodulate. So for instance a QPSK detector requires a lower C/N than a 256 QAM detector.
Since there is no coding gain in OFDM, C/N is equal to S/N, just need convert S/N to Eb/Nt for AWGN, then find the BER v.s S/N for different QAM modulation.