On Board Dianostics port
Its what dealers (ford in this case)/mechanics/theives use to plug in their devices and software to 'talk' to your vehicle
Theives will clone your key from drivers door or bonnet to unlock the van
They then plug their gadget into the OBD to code an immobiliser (all modern keys have immobilisers so that if someone tries to switch on an engine with an incorrect/unpaired immobiler the alarm goes crazy and engine wont start) to be able to drive off with your van with no alarms
In your custom, the OBD is where your right knee will be (ish) when sat down. Theres a piece of trim you can just pull and you'll see a plug
On mine, I've moved the OBD plug by unclipping it and a few wires, and then moving it; which will hopefully keep any potential theives looking for it long enough the alarm goes off (20secs I believe)