What is biasing?

"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.