dBm means "decibel milliwatt", this is more convenient to represent power.
If you have power in Watts - Pwatt, you can convert it to power in dBm:
dBm = 10 * log10( Pwatt * 1000 ) = 30 + 10 * log10( Pwatt )
If you want to get power in Watts from power in dBm:
Pwatt = 10^( (dBm-30)/10 )
dB is just a ratio in logarithmic scale. dBm means power ratio to 1 mW, represented in logarithmic scale.