This is the difference (or ratio) between two signal levels. In the case of fiber optic cable, we are comparing the power injected at one end of the cable to the power received at the other end.
If the optical power injected was -20 dBm and the power received at the other end -21 dBm, then the optical loss of the link would be -20 - (-21) = 1 dB.
Every time you double (or halve) the power level, you add (or subtract) 3 dB to the power level. This corresponds to a 50 percent gain or reduction. 10 dB loss corresponds to a tenfold decrease in signal level. A 20 dB loss corresponds to a hundred-fold decrease in signal level.
dBm (dB milliWatt)
This is the signal strength or power level. 0 dBm is defined as 1 mW (milliWatt) of power into a power meter. Small signals are negative. For example, typical LED power sources have an output power of -20 dBm where as Laser and VCSEL sources for fiber optic testing have an output power of -10 dBm.