A passive network comprising usually, but not always, resistors that reduces the power or voltage level of a signal without introducing significant distortion.
A potentiometer (pot) that is used to lower the amplitude of the signal passing through it. The amplitude can usually be set to any value between full (no attenuation) and zero (infinite attenuation). Pots can be either rotary or linear (sliders), and can be either hardware or "virtual sliders" on a computer screen.
The attenuator is used to attenuate a received signal. Reduction varys between models and is measured in decibels. Why would someone want to use this? It is usually used to reduce interference from intermodulation.