The explanation at
http://en.wikipedia.org/wiki/Decibel does a very good job of explaining the difference between dB (a ratio expressed in logarithmic form) and dBm (a ratio referenced to 1 milliwatt of power).
The SV-A15PRS is very similar to the Winegard and Channel Master distribution amplifiers. (HDA-100 & CM-3410). It will do fine, just keep water out of it.