The LM338's output voltage is determined by the voltage at its Adjust pin (the one shown at the bottom). Specifically, the output voltage is 1.24V (nominal) higher than the Adjust pin voltage. This 1.24V difference is generated inside the LM338 and is called Vref, the reference voltage.
The output voltage is affected by two circuit paths. The voltage divider made from R2+R3 and R1 sets the maximum output voltage.