A cabinet filler is a narrow piece used to close a gap between cabinets, or between a cabinet and a wall or another feature. It usually matches the cabinetry so the run looks finished. A filler can also be fitted to the shape of an uneven wall, a process called scribing. It is a separate piece rather than an extra storage cabinet.
In kitchen renovation planning, review filler positions on the cabinet drawings before the order is finalised. Ask how each gap will be fitted and finished, especially against an uneven wall. Check that nearby doors, drawers and appliances can open as intended. The width of each filler should be planned with the room, rather than treated as leftover space.