What is the Reference in Control Theory?
Imagine that you're heating (or cooling) a home with a modern furnace (or air conditioner).

the reference or set point is the temperature that you set your thermostat to be.  the feedback signal is the actual temperature that is measured with some kinda thermometer.

the actual value that you are trying to control, whether it's temperature or the position of a robot arm or the position of a pointer in an asynchronous sample rate converter, that actual value is compared to your reference value.  the difference between those two signals is what drives your "controller".

in the case of heating your home, if the actual temperature is below the reference (what you want your temperature to be), the furnace heat is increased.  if the actual temperature is above the reference, the furnace heat is decreased.