The term countersink can refer to a cone shaped hole carved for drilling, or to the tool that cuts these holes, which allow fasteners to lay flush with the edge of the surface in which they're embedded.
Common for drywall and furniture, countersinking creates a smooth look and reduces danger from edges. It can be achieved with bits on a standard drill, or using larger equipment like presses, milling machines, or lathes.
A frequent use of countersinking in industrial contexts is deburring—the process of removing raised metal edges that resulted from previous cuts, increasing safety and smoothness.
Proceeding slowly while creating a countersink hole can help you avoid chattering, the vibration from which can damage vulnerable surfaces.