Others could probably explain this better than I, but bias is sort of the operating voltage of the tube. Just as a combustion engine runs differently with different settings for gasoline flow for idle and running, tubes have differing requirements for bias. And there is a safe range of use. This switch sets the bias of the tubes at a setting low in the range, and a setting higher in the range.
In my amps the lower setting imparts a warmer, more relaxed sound and the higher a more dynamic, slightly more forward presentation.
A google search about tube bias should offer better explanations and information than I'm providing I'm afraid.
For example I found this on the page for the former production Decware amp, the SE84C+:
BIAS SWITCH
The switch as seen in the second picture in this manual exists for one reason only – to change the sound of the amplifier, giving you two possible sound signatures at any given point while listening. The switch changes the bias current of the input tube that results in a more laid back signature vs. a more pronounced dynamic and presence. No one agrees on which way sounds best – so it is a function of the source and or preamp you use as well as the type of speakeras you have that will determine which way will sound best to you.
The switch can be operated while listening without damage to the amplifier or speakers even though you will hear significant pop when the switch is changed. Don't worry this is normal. At 2 watts the amplifier is incapable of blowing up a speaker (even tweeters). On a similar note, the amplifier itself is impossible to blow up too. For example; shorting the speakers wires together will have no effect. Leaving the speakers unhooked from the amplifier, while usually death to most tube amps, will also have no effect. The output transformers are oversized, and 2 watts is not enough to damage them.https://www.decware.com/newsite/SE84CPLUS%20Owners%20Manual.pdf