The PT-3B is a 2.1mm DC power plug to 2 pin screw terminal adapter / converter.  These power adapters are typically used to connect the power input of an IP or CCTV security camera to a power cable pair, such as 18/2 power cable.  The 2.1mm female end conveniently connects to security cameras that have a standard 2.1mm male input.  The 2 pin screw terminal side makes it simple for the installer to insert and secure 18/2 power cable such as from industry standard RG59 Siamese cable typically used in CCTV installations.