What is biasing, regarding a tube amp?

What is meant when having to bias a tube amp?
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.
Nice description, Aball.

All I can add is that that fluctuation can sometimes be related to a tube aging, in which case it needs to be adjusted accordingly. Also, new tubes often go through a lot of change in the first 25 or so hours, so it's wise to do it at installation, then again soon afterwards.
Helpful so far, glad Mercedes asked. I've quite enjoyed my little ASL tube headphone amp, without really knowing what biasing and 'tube rolling' mean. I was simply assured that it was taken care of at purchase. So, now that we have the general definition, what are we looking for to make adjustments? Is there typically a knob or set screw on the exterior of a given amp or pre-amp?