Noise figure

Noise figure tells you how much a component dirties a signal by adding its own electronic hiss. Every amplifier, mixer, or cable has random thermal noise jittering inside it, and when a weak signal passes through, that internal noise piles on top — making the output signal-to-noise ratio worse than the input. Noise figure is exactly that degradation, in decibels: an ideal noiseless amplifier has 0 dB, a good low-noise amplifier maybe 0.8–2 dB, and a passive 3 dB-loss cable adds about 3 dB.

Noise figure matters most at the very front of a receiver, because of Friis's cascade rule: the first stage's noise figure dominates the whole chain, while later stages contribute far less (their noise is divided by the gain ahead of them). That is why a satellite dish or 5G base station puts a low-noise amplifier right at the antenna — every tenth of a dB there directly limits how faint a signal the system can still hear.