This is a torque wrench. It tightens bolts and nuts using the drive on the end, which takes any sockets and accessories.
What makes this a special way to tighten fasteners is a scale that shows how tightly the nut or bolt is being done up.
This head bolt is being tightened to a specified torque recommended by the manufacturer, as being tight enough to ensure that the nut won't come loose, and the parts are being held together firmly, but not so tight as to risk breaking the bolt or stripping its threads.
Manufacturers don't specify torque settings for every nut and bolt, but when they do, it's important, and that's why the torque wrench is such an important tool.
