Milling is the mechanical processing of printed circuit boards with a rotating cutting tool. This allows you to realize round outer contours (contour milling), slots and breakouts, e.g. to create space for a mechanical connection or to make breakouts, without or with metallization, in the PCB (sideplating).
As with all cutting processes, material is removed from a blank in the form of chips, in PCB production mostly FR4 (or aluminum). The edges of the finished printed circuit board are therefore never completely smooth due to the production process, but have a certain roughness.