For Raspberry with 40 pin GPIO, this stacking Header allows to use directly and easily shields/cards/hats planned for Raspberry Pi 2 and following :-).
2x 20 pins connector for the Raspberry-Pi's GPIO (or the N2)
Stacking several shield/plate, breakout, etc on a Pi is not easy if you don't have a proper stacking Header. This Pin Header is cut for the Pi B+ with its super high female connector (to fit on the GPIO 40 pins of the Pi) and extra long pins (male) to connect a ribbon, another "shield", a Pi-Cobbler PLUS, etc.
With this connector plugged in the GPIO of the Pi, you are just above the USB connectors (~0.5mm) and above the Ethernet connector (~1.5mm). The Jack plug is even lower.
The 2 x 20 pins are extra long, 9.7mm, which means that there is still 8mm when a card (1.5mm thick as standard) is "installed" on the stacking Header... So what to connect another connector :-).
Contains: only the stacking header, sold without Raspberry Pi used for illustration purposes
26 or 40 pins - know how to make the right choice
This 40-pins stacking Header is perfect for your Raspberry Pi 2 & Pi B PLUS. There are however cases where a 26-pins Stacking Header (Pi B PLUS compatible) will be more suitable. Take the example of an interface card Pi B (thus 26 holes) which works on a Pi 2 (or Pi B Plus) and behind which you would like to connect your mounting Cobbler or Perma-Proto Pi made with your old Pi. In this case, it will be wiser to opt for a stacking header 26 pin.