Bias is the power put into a tube to push the electrons across the vacuum in the tube. Too little and the tube is weak, too much bias and the tube may be damaged.
(and for you experts out there, STOP! he wants real simple explanation. not your twelve page essay.)
The real tubes needing a careful watch on bias are power tubes.
Preamp tubes usually can be OK with a fixed bias. And almost never need to be checked for bias.
So you DO have to learn how to adjust the bias IF you are buying a tube amp without any autobias.
Or you will have to pay regular visits to your local tube guru.
