Why A receive attenuator is a switchable RF pad (commonly 6, 12 or 20 dB) inserted ahead of the receiver's first amplifier or mixer. Very strong signals, such as loud locals or high noise on 160 and 80 meters at night with a big antenna, can drive those stages out of their linear range, producing distortion, spurious mixing products and phantom signals across the band. Knocking the whole RF input down a fixed number of dB keeps the front end linear; since both the desired signal and the noise drop together, the signal-to-noise ratio is essentially unchanged on strong signals.
Watch out Reducing an excessive audio level is the job of the AF gain control or the AGC, which act after detection; the attenuator works on RF before any amplification or mixing, so it also cures the distortion that loud audio cannot.
Attenuator = RF pad at the antenna input. Protects the front end, not your ears or your amplifier.