By definition, it means 'to diminish'.An example of attenuation is when earplugs are worn to sleep, where the volume of sound around is greatly reduced when passing through the foam material.
When a signal travels through a medium, its energy is diminished, resulting in a lower amplitude of the wave.This loss in amplitude is what is defined as attenuation and is measured in decibels.
A decibel (dBs) is a measure of the increase or decrease in the amount of power between two points A and B (see diagram).
As electromagnetic frequencies pass through bloqed clothing and apparels, the energy is greatly reduced or attenuated.
Source: NASA, EPA, Howstuffworks.com