If you look at the topology of class A and class AB amplifiers you will find that they are essentially the same. So why does one require huge power supplies and heatsinks and the other doesn't. The answer is in the way the output transistors treat the signal they are carrying.
Let's start at the beginning. An audio waveform (sinewave or music signal) has a negative and positive going wave which oscillates around a zero point. The amplifier output stage consists of a pair (or multiple pairs) of transistors. One transistor amplifies the negative part of the waveform and the other amplifies the positive part. Each transistor takes an input for it's half of the waveform at it's base connection. It then reproduces a larger copy of that by taking energy from it's supply rail through it's collector connection (1 positive and one negative) and produces a proportionate voltage at its emitter connection via a resistor and send that to the loudspeaker. So the loudspeaker gets the whole waveform, half from the negative transistor and half from the positive transistor. With me so far? Good.
Now, the problem arises in that each transistor will not start to conduct right from a zero signal. It will start to conduct when the base signal reaches about 0.7volts (or minus 0.7 volts for the negative side). This would give a small step in the output waveform every time the input waveform crosses the zero region of the waveform. This is how a class B amplifier works and it sounds terrible. To get over this problem the input signal to the transistor is superimposed on a small DC signal which prevents the input to each transistor from dropping into this 'switched off' region. This is called output stage biasing and is usually adjustable.
Power transistors are current amplifier devices and the output biasing is actually measured as a current not a voltage. I mentioned previously that the output from the transistors go to the loudspeaker via a resistor. This is called the emitter resistor and it is there to ensure that the transistor always has some load, otherwise it would become unstable. As all the output passes through these resistors, they have to be quite large. As the output current passes through this resistor a small voltage is developed across it. When the whole amplifier has no input signal, the only current passing through the emitter resistor will be from the DC bias. This current passing will develop a small voltage across the resistor. It is this voltage that you are measuring at the bias test points. The setting is for bias current, but you cannot measure that without disconnecting an emitter resistor and putting an ammeter in series with it. But, using Ohms law and a bit of maths you can work out what the bias current is by measuring the voltage drop across the resistor. What the manufacturer will do is to give the voltage (in millivolts) that they consider gives the best minimum emitter current to ensure that the output transistors are held just outside of that zero switch on point.
If you decide to increase this given value you will effectively be converting your amplifier into a class A unit. The higher the bias current, the higher the minimum current the transistors will pass - remember that current equals heat. A class A amplifier usually biases it's output transistors to half the supply voltage and this means that their current is always running at maximum. This creates a lot of heat which is why class A amps need large PSUs and heatsinks.
O.K. That will do for now.