A brake pad spreader is a tool used when replacing worn disc brake pads. As brake pads wear down, they get thinner, and the hydraulic caliper pistons push further out to keep the pads close to the rotor. When you install new, full-thickness pads, there is not enough room to slip the caliper back over the rotor. The spreader pushes the caliper pistons back into the caliper body, clearing the space needed for the new pads.
The tool typically consists of a flat plate and a threaded bolt with a handle, or a ratcheting spreader mechanism. You place a worn brake pad against the caliper piston to protect it, insert the spreader, and turn the screw or squeeze the trigger. This pushes the piston back slowly and evenly. When compressing pistons, keep an eye on the brake fluid reservoir under the hood, as the returning fluid can overflow if the system was topped off recently.