What is biasing?

Hey, I got it--- It was clear as mud.

"Bias" in any context means something which is a steady value, on top of which some variation occurs. For a vacuum tube, bias current means the current that flows through the tube (from the power supply to ground via a beam of electrons in the tube) when the control signal (applied to the grid of the tube) is zero. This background current flow is set at a level where the tube works best by application of a bias voltage (DC) to the control grid. The audio signal varies the voltage on the grid, which varies the current through the tube, which also flows through the primary winding of the audio output transformer, which generates a voltage in the secondary of the transformer, which goes to your speakers...whew! Adjusting the bias is a ritual solemnly performed by some audiophiles. There are also some amplifiers that automatically adjust their bias, but this takes the fun out of life.

I like Rushton's answer. Simplistic works best for me when it comes to hot and cold running electrons. Eldartford's answer was far too correct! (Actually very helpful, thanks.)

We just biased our new tube amp for the first time, manually. Really, really exciting moment. There's nothing like dragging 65 pounds of iron out of the rack just so you can get to it with a multimeter. Auto-biasing must be like auto-return tonearms, keeps you from truly bonding with your stereo.

One possibly useful tidbit: if the amp has more than one output tube per channel there may be a facility for balancing the tubes against each other as well as biasing the overall DC voltage level. Unbalanced tube voltages on the same channel apparently feeds DC into the output trannies, which tends to saturate them. (Eldartford can correct that if I messed anything up.)