Biasing a transistor
Biasing generally refers to the DC operating point in quiescient state.  There is therefore no AC to shift or amplify.

The point of biasing something like a audio amplifier is to put the operating point near the middle of the range the circuit can do.  The incoming AC signal then adds and subtracts from that.  With the quiescient operating point in the middle, the amplitude of the AC signal the circuit can handle is maximized.  The output signal is no good when it clips against either end.  With the operating point in the middle, the nearest distance to clipping - in either direction - is maximized.