Bias is a DC Voltage that is placed on the "input element" of the amplifying device. (For Tubes, this is the Grid, for Transistors, it is the Base)
This allows the tube/transistors to amplify an AC signal without destroying the waveform. Basically, without bias, the tube/transistor becomes a diode, and rectifies the signal - you don't want to do that.
Let's say the AC signal is a line-level input of maximum 2V AC. This means that 2V of electricity flowing in both directions. Bias turns the AC into Varying DC, so the tube won't badly distort the signal.
Bottom line: Bias sets the tube/transistor up so it can amplify a signal without chopping it up badly.
On Tubes, there needs to be a way to adjust the bias, as a new set may need slightly different bias for even the same model tubes. Transistor bias is usually fixed.
This allows the tube/transistors to amplify an AC signal without destroying the waveform. Basically, without bias, the tube/transistor becomes a diode, and rectifies the signal - you don't want to do that.
Let's say the AC signal is a line-level input of maximum 2V AC. This means that 2V of electricity flowing in both directions. Bias turns the AC into Varying DC, so the tube won't badly distort the signal.
Bottom line: Bias sets the tube/transistor up so it can amplify a signal without chopping it up badly.
On Tubes, there needs to be a way to adjust the bias, as a new set may need slightly different bias for even the same model tubes. Transistor bias is usually fixed.