The LM358 is operating as a differential amplifier, with the feedback path going through D12, U2, U1, TX1, and the rectifiers and output filter. It modulates the primary such that the voltages at its two inputs are equal. The voltage at the left of R12 is lower than that at the right. The wiper of pot VR1 is adjustable between that lower voltage and the top of D13, which is 0.6 V higher. So U3A is comparing the voltage at the low side of the load to a voltage that decreases across R12 in direct proportion to the load current, plus an adjustable percentage of 0.6 V. Because R12 is 1 ohm, the math is simple; 0.6 V equates to 0.6 A, the max output current available if the pot is all the way to the left.
BTW, the pot pin numbers probably are incorrect. I don't know if this is a true standard, but it is ((very)) common practice for trimpots to be numbered like this:
1 - Counter-clockwise terminal
2 - Wiper
3 - Clockwise terminal
For this circuit you want the clockwise terminal connected to D13, so that turning the pot clockwise increases the output current.