The Signal-to-Noise Ratio (SNR) is defined as the ratio of the power of the desired signal to the power of the background noise: $SNR=\frac{P_{signal}}{P_{noise}}$. A higher $SNR$ means the desired signal is much stronger than the noise, leading to clearer reception, lower error rates, and better overall quality. In audio, this means less 'hiss'; in data, it means fewer bits are flipped.