Can anyone explain to me what this actually is and does, and how much it would cost when it needs refilling. letters not necessarily in the right order

It is urea (pig urine to be exact) and is used to lower emissions. It is standard on all diesels for 16MY. Top ups are covered as part of the service and this should be enough of an interval for most users. Outside of this, your dealer can top up but unsure of costs. It is mandatory to have ad blue to start the engine and the vehicle will not be allowed to start if it runs out. You will be warned at various intervals though.

There are many forecourts stocking Adblue in small quantities if you use a tank full before service & it is relatively straight forward to top up - any spillage leaves a crusty residue
My previous Merc S350 used a tank full just within the service period but I was able to easily top up (full) from our own stock for running wagons - always had a problem with the dolly bird at Merc service telling me that they had done the Adblue and me telling them it must be leaking then and to knock it off the service price