The SNR measures Signal-to-Noise Ratio in dB after digital matched filtering operation inside the demodulator. For 8VSB, we consider any number greater than 20 dB is a good set up.
If SNR is less than 20 dB, then the unit is seeing too much noise, co-channel interference, or adjacent channel interference. If the noise is the issue, then add an LNA amplifier to increase power level. If co-channel interference is the issue, adjust antenna pointing to find a working case.