It is a ratio between two values. The decibel is a "dimensionless unit" like percent.
In acoustics to quantify sound levels relative to some 0 dB reference. The reference may be defined as a sound pressure level (SPL), Commonly 20 micropascals (20 μPa) which is the softest sound human can hear.
In case of sound:
In case of light:
The dBm calculation method is: dBm = 10 log ( P1 / P2 )
where
P1 = measured power level ( e.g. in mW )
P2 = reference power level, which is 1 mW - Maximum intensity of sunlight at ground level (directly overhead, no smog, etc.) = 1 kW/m2 or 1 mW/mm2.