The signal to the speakers alternates from positive to negative with respect to ground. To increase efficiency in a push pull power amp, the positive half of the signal is amplified by one half of the tubes, and the negative half is amplified by the other half. This means that half the tubes are off about half the time. At the point where the signal is passing from positive to negative, or back, one half are in the process of turning off while the the other half are turning on. If the bias is too low, the turning off half will turn off before the turning on half can turn on, that is, they will both be off during the transition. This is called crossover distortion and it is not pleasant. In other words, both halves need to on slightly even with no signal applied.
If the bias is too high, the tubes will have too much current through them while idling, they will run hotter, and this will shorten their life.
In low level settings like pre amps, the bias is usually set by the manufacturer and is not user adjustable. These tube are run class A and amplify the signal through both the negative and positive half of the cycle. It would be difficult to change this bias without knowing what you are doing. If you have to ask, don't try it.