What does this component do in this series pass regulator
It supplies current to the zener diode which acts as a reference voltage to the emitter of Q2. The 470R and 56OR form a voltage divider that produces a signal at the base of Q2 (6.9V). This is compared with the zener voltage (6.2V) which is lower by the base -emitter voltage of Q2 (0.7V). When this condition is met the output will be 12.7 volts. If the output is pulled lower (large load current) more current is allowed to go into the base of Q1 (less through Q2) and raise the output voltage to the desired level (12.7V)