A decibel - the most awesome thing in the world.
Contrary to popular opinion, the decibel is not exactly a pure
measurement. It is a multiple - amplifying a sound by two decibels multiplies the
frequency volume twice over, amplifying it by seven or more may blow your speakers, unless you have kickass speakers. On the other hand, putting the amplification to a negative number will reduce that
frequency by the multiple.
Adding decibel amplification usually comes in the form of a number with a + for adding amplification or a - for reducing it.