Represent gains or attenuations logarithmically (base 10) (the Bel)
But to make numbers more convenient, scale by a factor of 10 (the deciBel or dB)
G = 10Log(P out / P in ) in dB
An amplifier has a power gain of 1000.
What is this in G = 10Log(1000) = 10 x 3 = 30 dB
An attenuator has its output power 1/10th of its input is its transfer function in dB?
G = 10Log(1/10) = 10 x -1 = -10 dB. (Note -dB can be negative)