A ridge reamer is a tool that cuts away the lip of carbon and metal at the top of a cylinder. As pistons travel up and down, they wear away the cylinder walls, but the piston rings never reach the very top. This leaves a raised ridge of metal and hard carbon just below the cylinder head surface. If you try to push the piston up and out of the block without removing this ridge, the top piston ring will catch on the lip, breaking the ring lands or seizing the piston.
The tool mounts inside the cylinder and uses an adjustable carbide blade. You turn the lead screw to expand the guide blocks against the cylinder walls, then rotate the tool with a wrench. The blade shaves away the ridge until it sits flush with the rest of the bore. Work slowly and check your progress often. Shaving too deep will gouge the usable cylinder wall below the ridge.