This machine has two uses - for shrugs as shown in this video and for deadlifts. The main muscles that a shoulder shrugs target are the trapezius (traps) muscles.
To start, make sure you have a straight back, knees slightly bent and grip both handle bars on either side. Lift up, squeezing the glutes.
Once stood up, you want to focus on activating the traps (trapezius muscles). Pull up keeping the arms straight and not bending your elbows. Ideally holding and squeezing at the top and coming back down in a controlled movement.