dB is a
decibel which is a logarithmic representation of the power gain ratio or power loss ratio of any circuit (in this case a filter). It is equal to 10*Log (Po/Pi) or 20*log(Vo/Vi) where Po and Vo are the output power or voltage values and Pi and Vi are the input power or voltage values.
Strictly speaking you can only use the voltage version of the formula if the input and output impedances are identical but typically any impedance differences are ignored.