A brake caliper wind-back tool is a specialty press used during brake pad replacements. On front calipers, you can compress the piston straight back using a simple C-clamp. However, many rear calipers have an integrated parking brake mechanism. These pistons use a threaded internal screw. To compress them, you must turn the piston while pushing it inward. The wind-back tool uses a threaded rod and adapter plates to do both at the same time.
The tool adapter plates have small pins that lock into the slots on the face of the caliper piston. As you turn the tool handle, it applies pushing force while rotating the piston down the threads. Trying to do this with needle-nose pliers is frustrating and can tear the rubber dust boot. Once the piston is fully retracted, make sure the slots line up with the locating pins on the back of the new brake pads.