Tubes can only increase the input signal in a positive direction in relation to ground. The amplitude of the output signal will fall somewhere between ground and the B+ voltage. Since the input audio signal is AC, and has both a positive and negative portion in relation to ground, a DC voltage offset is used so the AC waveform can "fit" inside the tube's positive operating range. This offset is called the bias voltage.
Bias voltage can be applied from a voltage source (fixed bias), usually in the power transformer, or developed by the cathode of the tube (cathode bias). There are a few other ways, like using a zener diode (which is not too common), and mixtures of the two methods like Fender did, which is not well appreciated, so the subpages below stick with fixed and cathode bias.
Note that fixed bias can be variable. Some amps have a pot that allows the bias voltage from the transformer to be varied. Cathode bias is usually adjusted by swapping out the cathode resistor. A pot doesn't work here because the power requirement is high and needs a power resistor, like a multi-watt cement type.