Actually it is not db it is dBm
The difference between dB and dBm is;
dB is relative to a specified unit, and also dimensionless because values are divided (watt to watt for example)
dBm is absolute value and “m” is for “microwatt”
there are more decibel units, for antennas dBi is used and here i is for “isotropic”, gain of antenna is defined relative to isotropic antenna.
so minus corresponds a watt value actually you can calculate it or convert and find corresponding value.
decibel is used because using “watt” makes things complicated so many digits so decibel makes things simple