Signal to noise ratio (often abbreviated to SNR or S/N) is the ratio of signal power to the noise power corrupting the signal.
The ratio compares the level of a desired signal to the level of background noise. The higher the ratio, the less obvious the background noise will be.
Signal to noise levels are often stated in measurement of dB SPL. The larger the number the bigger the difference is between the audio signal and the noise floor, so the lower the perceived noise.