LD1117v33 voltage regulator
Vref, reference voltage only matters when using the adjustable version of the LDO in question. For LD1117v33 it is irrelevant. By the way if you check the datasheet, then you can see what Vref is on figure 2.



Vref = 1.25 V and Vref is the voltage across R1. The LDO adjusts its output until the voltage across R1 is Vref, thus you can set the output voltage to a desired value with an appropriate voltage divider.

Now, in case of the LD1117v33 the output voltage is fixed 3.3 V. There is no  ADJ pin, just GND and you do not have to care about the feedback externally. You only need the input and output capacitors.