It tells you what it is for in the listing."For sale is a Atari 5200 digital controller interface that lets you use controllers from the Atari 2600, Sega Master System, or Sega Genesis. Built-in Atari 5200 keypad with start, pause, and reset."
As mentioned in the listing you posted (you really should read those), the 5200 controller port is 15-pin. This is a very nice accessory actually, just because it enables you to use a controller that's not complete crap.