The dBm System
The dBm system tells you "how many multiples of 1mW are in my signal?"
Since 0 dBm is defined to be 1 milliwatt, then 30 dBm would be 1,000 milliwatts or 1 Watt. Each step of 30 dB added to your signal power increases the power represented by a factor of 1,000.
So 30 dBm is 1 watt, 60 dBm is 1 kilowatt, and 90 dBm is one megawatt, and so forth.
If the dBm value is negative, then instead of multiples of 1mW you are dealing with fractions of 1mW. Each step of 30dB subtracted from your signal power decreases the power represented by a factor of 1,000.
So -30 dBm is 1 microwatt, -60 dBm is a nanowatt, etc.
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