An edger is used in the finishing process of concrete construction. To properly use an edger you must not have concrete too dry or too wet. An edger can make the difference between a very quality looking pour and an unfinished looking job. This tool should be used carefully as it can dig in and gouge the concrete. they can come in handheld versions or with a place to attach an extended handle to get into places that are hard to reach.

Images from: https://www.jimslimstools.com/Products/Kraft-CC351-Walking-Concrete-Edger-8in-x-5-38in-Radius-12in-Lip__CC351.aspx and https://www.lowes.com/pd/Marshalltown-Blue-4-in-x-6-in-Steel-Concrete-Edger/1000107171