Bias adjusts the operating point of the tube (or transistor which has the same thing). Tubes and transistors don't operate from zero voltage but rather hundreds of volts or a few volts respectively, depending on the circuit. The bias is this voltage level needed for the tube or transistor to amplify the signal adequately. There is an upper and lower limit to the setting and it can flucuate over time which requires "rebiasing" to get it back at the right point.
It is like the idle engine speed in your car. When you start your engine, it will idle at a low speed - more than zero but not too high so you don't run out of RPM range. Same goes for amplifiers.
It is like the idle engine speed in your car. When you start your engine, it will idle at a low speed - more than zero but not too high so you don't run out of RPM range. Same goes for amplifiers.