The MOSFET Plug is a small board containing two high-power MOSFETs for switching loads at up to 5A @ 50V (this will require an extra heatsink).
The Kit version comes with all the parts needed, including a high-power terminal block with individually detachable connectors. Note that no 6-pin headers are included.
The PCB only variant contains just the board for the MOSFET Plug. No parts, no headers.
As connectors you can use a Plug Headers set with 10 male and 10 female 6-pin headers, which makes it easy to connect multiple plugs and JeeNodes in a “standard” way. Or you can solder the board directly in place for more permanent use.
The documentation and full design details are freely available.