Which part, in particular, is confusing?
"BluetoothReceiver::BluetoothReceiver() : bandpassFilter(BPLENGTH), differentiatorFilter(DIFFLENGTH) {...}" is the default constructor. The stuff after the ":" is the initialization list and may be used to initialize member objects or base class portions of a derived object.
My guess is that bandpassFilter and differentiatorFilter are both contained by BluetoothReceiver and are initialized with BPLENGTH and DIFFLENGTH upon construction.