R2 and R3 form a voltage divider across the reference voltage, which leaves 76 mV across R2. In other words, the op-amp's non-inverting ("+") input is set to 76 mV less than the voltage at the LM338's output.
The op-amp compares this voltage with the voltage at the right hand end of R6. When the output current is high enough that 76 mV is dropped across R6 (this occurs at an output current of 380 mA), the op-amp starts to pull its output low.
This reduces the output voltage, which will cause the current drawn by the battery to drop. The circuit reaches an equilibrium where the output voltage is just right to cause about 380 mA of current to flow into the battery.