active buttons activate a pad. This drops the signal a little bit so that a bass with active electronics or pickups doesn't overdrive your preamp. The pad's value is expressed in decibels. Usually most are -10dB.
I own a RBI and the active button on the front switches on and off the "sansamp" circuit. yes, the volume drops to the input level from guitar or preamp send. a friend told me that the only reason they have the button on the rbi is to remind you how your guitar sounds WITHOUT the RBI. seems true enough for me!