Senario: When you are sailing a real sailboat and you are the skipper you have a hand on the tiller. With that feel you can tell how much resistance the rudder producing as the boat moves through the water. To keep the boat on course sometimes you feel a considerable resistance as you push the tiller to one side.
As a radio control sailor you don’t feel any of that. There is no feedback system to tell you if your just dragging your rudder sideways though the water until you see that your boat is slowing down.
So, what you see in the picture is a resistor in parallel with the servo power lines. The arduino analog input is measuring the voltage across the resistor. So basically its an ammeter measuring the servo current draw.
The relative current number is sent back to the hand held transmitter.