During a pad change you must push the caliper piston back into its bore to make room for the thicker new pads - and a caliper compression tool makes that job easy and safe. Here is what it does and why some calipers need a special version.
Why you compress the piston
As pads wear, the caliper piston extends to keep contact with the rotor. Fresh, full-thickness pads need that piston pushed all the way back in, or they will not fit over the rotor - a top reason a new pad won't fit. The compression tool applies even force to retract the piston without damaging it.
Front calipers: push straight in
Most front calipers have a piston that simply pushes straight back into the bore. A C-clamp or a basic caliper piston tool presses it in evenly. Press slowly - the displaced fluid pushes back up into the reservoir, so watch the level so it does not overflow; see can you overfill brake fluid.
Rear calipers: often must be wound in
Here is the catch: many rear calipers with an integrated parking brake have a piston that must be TWISTED (wound) clockwise while pushing, not just pressed. Force one straight in and you damage the parking-brake mechanism. A rear caliper wind-back tool with the correct pin pattern turns and presses at once - this is why a special tool matters.
Doing it right
Use the correct tool for front (push) vs rear (wind), retract the piston fully, keep an eye on the reservoir, and never pry with a screwdriver against the piston face. With the piston retracted, quality pads and fresh hardware go in cleanly.