Please explain

Is there a simple explanation available for "Bias" in relation to amps (especially SS if they are different). I notice posts refering to adjusting Bias - why? what is accomplished? Is there perhaps a web location that explains a lot of the basics that would be useful to those still learning?

I've looked thru the archives but didn't notice aanything that answered my question (I admit I could have missed it).

I thank all of you for your help