This is an RP-SMA plug to SMA jack adapter. The FreakLabs radio boards come with a standard RP-SMA jack connector mounted on the PCB (if the external antenna option is chosen). If you'd like to connect the boards to antennas with an SMA plug connector, then you will need this adapter.
The distinction between RP-SMA plug/jack and SMA plug/jack is quite confusing. If you have any questions regarding connecting to external antennas, you can email me through this site or post your question on the forums. I've also included pictures of both ends of the adapter so that you can visually confirm if its what you will need.