When you looked at how a common collector transistor amplifier works, you noticed that most of the behavior is controlled by the voltage applied at the base. Most of the time, the signal we want to amplify is a signal that alternates between positive and negative.
Since the transistor needs at least enough voltage at the base to overcome the base-emitter junction voltage (0.7v typical for silicon transistors), any voltage below that will drive the transistor into cutoff, clipping and distorting the signal.
One way to overcome this and allow negative signals to be amplified is to set a constant voltage at the base that will be varied up and down by the alternating signal to be amplified.
The setting of that constant voltage at the base is called biasing of the transistor.
The easiest and most common way to bias a transistor is to use a two resistor voltage divider. As you saw in a previous lesson, the voltage divider has the weakness that anything connected to it will "load" the circuit, and change the voltage across the output resistor.
When used to bias the transistor, the voltage divider will never get into a loaded situation, since the base will draw very little current.
