The reflux loop is either a flow or temperature controlled loop and is used to adjust the purity of the overhead product.
Distillation efficiency depends on the amount of contact between the vapor rising and the liquid falling down through the column. Therefore, some of the overhead liquid product is sent back or refluxed to the top of the column. This assures that fewer of the heavy components come off the top of the column, thereby improving the purity of the overhead product.
A poorly operating reflux control valve will cause product purities to oscillate, make the column difficult to control and less efficient operationally.