For example, one watt is 1,000 times bigger than a milliwatt. That means that I can tell you to start with a milliwatt, and make it bigger by 30 dB (since 10³ = 1,000).
That convention is handy enough it often just gets written as "dBm" meaning decibels referenced to milliwatts. You may have even seen dBm listed on your phone as a kind of signal strength.