An attenuator is a device that reduces (or "attenuates") the level of a signal. A fader on a mixer is an attenuator. Attenuators are particularly useful for adjusting how much "depth" of modulation you send from one module to another module's parameter – for example, whether an LFO makes the pitch of a VCO go up and down a lot (the result with no attenuation) or a little (the result of attenuating the LFO's signal).
