Adding my 2 cents' worth, the input to the PID controller is not the position, but the position converted into a current or a voltage.
To express in words the salient points to help in answering the question, what the other respondents have explained using mathematics:
Essentially the electronic PID controller 'sees' at the input for the desired mass position Set Point a voltage or a current. At the other input for the feedback sensor monitoring the position of the mass, the PID controller 'sees' a current or a voltage converted by the feedback sensor.
The mass position feedback sensor is not explicitly shown in the provided diagram, so one may assume it's the unity feedback line going from the output of the System Dynamics' block to the negative input of the summing junction.
The unity negative feedback is to signify that when the user's desired Set Point matches the mass position, the feedback signal from the position sensor has the same magnitude but opposite sign, to the Set Point signal.
The electronic PID controller then electronically subtracts the user's desired mass position converted current or voltage value, from the actual measured mass position by the feedback sensor's converted current or voltage value.
The difference between the user's desired converted current or voltage value and the feedback sensor's converted current or voltage value may be referred to as the Control Error or CE for short.
If the CE is zero, the PID controller does not output any current or voltage; i.e. the PID controller's output is zero because the mass position is at the user's desired set point.
If the CE is not zero, either a positive or negative value, the PID controller will output either a negative or positive voltage current or voltage to adjust the mass' position to match the user's desired set point. The negative or positive current or voltage drives an electric motor to adjust the mass position. When the mass position is shifted to the user's desired set point position, the CE is zero and the PID outputs zero current or voltage, to stop the mass from moving.
The electric motor converts the electrical current or voltage provided by the PID controller, into a mechanical Force or Torque (twisting Force) to move the position of the mass.
The negative current or voltage output of the PID controller may be arranged so that the electric motor will turn, say anti-clockwise, resulting in the mass moving to the left (or down depending on your point of view), and the positive current or voltage output of the PID controller may be arranged so that the electric motor will turn clockwise, resulting in the mass moving to the right (or up). The convention for left or right, or up or down, is adopted depending on how the mechanics of the plant is configured.
Disclaimer: Please correct any misinterpretations I may have made in the text above; I would appreciate constructive criticism.