Bias determines the amount of current that flows through the output tubes at idle with no input signal present. It is determined by the voltage differential between the cathode and grid of the output tubes. A detailed explanation of how a vacuum tube works is outside the scople of this article, but  a much simpler explanation follows. The grid of the tube acts as a valve and controls the flow of current through the tube. Changing the voltage on the grid changes the amount of current flow. All output tubes are rated for a maximum current disipation and bias should generally be set at 60 to 70 percent of that maximum. The lower the number the colder the bias and the higher the number the hotter the bias. If you prefer a very clean sound with a lot of clean headroom you will probably prefer a somewhat colder bias. Otherwise, a hotter bias setting may work best for you. I prefer a bias setting of approximately 70 perent for many amps. Actual bias is determined by measuring the plate current disapation and the plate voltage either directly or indirectly. Once these are known the actual bias can be computed with a simple formula.