Passive means, that each unit is made of two sockets and a 50k linear potentiometer only. There are no active parts like amplifiers or buffers and no power supply is required.  

This is how the A-183-1 works: At the output socket appears the attenuated input signal, i.e. zero level at the fully CCW position and max. level and the fully CW position. Applications: Adjustable level of CV or audio signals to control other modules that do not have attenuators (e.g. Dual VCA A-132-1, Quantizer A-156, CV-to-Midi interface A-192 and modules of other manufacturers).