Bias is the operating voltage for a vacuum tube, which need to be adjusted at times. Does not apply to solid state amps, as far as I know.
On a solid state amp, the bias ajustment determines how "enriched" in class A-B the amp runs. The higher the bias, the longer the amp stays in class A. I built a Hafler DH200 about 25 years ago and the bias is turned up to about 3 times the stock setting. The amp runs real toasty and sounds much better. It is still working fine in my daughter's system. As I recall, I removed the rail fuses and clipped a digital meter across the fuse terminals while adjusting the bias pots. This is NOT a recommended mod for all amps as some of them will go up in smoke. Don
In solid state amps there is an adjustment commonly called "bias". It does not set a level of current, as in a tube amp, but it equalizes current from the complementary set of output transistors so that when there is no signal being amplified the amp output is nearly zero volts. If the amp puts out voltage unrelated to the audio signal, the loudspeaker voice coil is heated, and the cone is pushed away from its rest position so that its available range of motion is reduced.