jerk blocks also called a jerk box or olympic weightlifting technique box, is a stackable, raised platform to get the barbell a few feet off the floor so you can practice jerks. A jerk is the final portion of the clean-and-jerk, moving the bar overhead

For that matter, jerk blocks can be used for other standing barbell movements, such as push presses, strict military presses, and squats that start at the bottom of the range of motion.