Disc brake pads for a caliper with 1 piston. The pads clamp the brake disc and slow the wheel; they are chosen to match the shape and type of the caliper.
Replace them when the pad material is worn to the minimum, the brake feels "dead," squealing and grinding have appeared, or grooves are visible on the disc. Replace the pads on an axle as a pair and compare the shape with the old ones: the number of pistons and the mounting "ear" must match.
