the signal-to-noise ratio

Think of trying to hear one friend speaking at a loud party. What matters is not just how loudly they talk (the signal) but how loud they are compared to the crowd's roar (the noise). The signal-to-noise ratio captures exactly that: how much your wanted signal stands out above the unwanted background. When the friend is much louder than the room, you catch every word; when they barely rise above the din, you mishear and have to ask again.

Formally, SNR is the power of the signal divided by the power of the noise: SNR = S/N. Because real systems span a huge range, engineers usually quote it in decibels: SNR(dB) = 10 times log10(S/N). A ratio of 1 (signal as strong as noise) is 0 dB and is dreadful; 10 is 10 dB; 1,000 is 30 dB and is good. The higher the SNR, the more confidently a receiver can tell which symbol was sent, and so the more signal levels you can safely use — which is exactly the S/N term inside Shannon's capacity C = B log2(1 + S/N).

SNR is the single most important quality number for a channel, because it directly sets how fast you can go. Anything that weakens the signal (attenuation over distance, walls between you and the Wi-Fi router) or adds noise (interference, thermal noise, a neighbour's microwave) lowers SNR and forces a slower, more robust modulation. This is why moving closer to the access point, using a shielded cable, or switching to a cleaner radio channel can speed you up: you are not adding bandwidth, you are improving SNR.