Refers to the dBm power level scale. Referenced to 0.775 volts in 600 ohms at one milliwatt. dBm only used for 600 ohm circuits.
Used to compare gain or loss in decibels in power functions (such as amplifiers, etc.) Like all decibel functions, expresses a relationship not an absolute value. See decibel for more. As a logarithmic scale if the amplification of an amplifier is increased from 25 dBm to 28 dBm, that increases the amplification by  2x.  Halving the power (1/2) subtracts 3 dBm.