The circuit is a basic single OP differential amplifier. Vref respectively GND are different output references, chosen according to the intended usage.
Vref is for bipolar input current/voltage range with an unipolar ADC (e.g. 0 to 3.3 V ADC range, output = 1.65 V with input zero). GND is for unipolar input range.
The 0-ohm resistor is used as "net connector". SHUNT_LOW_SUM is connected to DC- net, but should be wired separately between shunt and amplifier input to achieve a 4-wire (kelvin sense) connection, avoiding errors due to DC- net voltage drops. The 0-ohm resistor is a trick used with simple PCB layout systems that don't support star points or net connectors.