Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium.
That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat.
To compensate for this loss, amplifiers are used to amplify the signal.
To show that a signal has lost or gained strength, engineers use the unit of the decibel.
The decibel (dB) measures the relative strengths of two signals or one signal at two different points.
Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified. Variables PI and P2 are the powers of a signal at points 1 and 2, respectively.