Edger
A hand tool that rounds the edge of a slab where it meets the form, producing a compacted radius that resists chipping.
What does edger mean?
Edging compacts the concrete along the perimeter, which is the most vulnerable part of a slab to spalling under impact. The rounded profile also sheds a form line that would otherwise be a sharp, brittle arris. On decorative work the edge radius is chosen for appearance as much as durability.
In practice
That smooth rounded lip along the side of a sidewalk is edging, and slabs without it chip along the perimeter within a few winters.