What does it mean when in a recipe it says 400g king prawn tails, shelled and deveined?

It means without heads, shells, and their 'veins', The 400g needs to be the weight of the 'prepared' prawns,  not the 'whole' ones.

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"shelled" means that you have ot remove the shell, if it hasn't already been removed.  Some prawn are not shelled when you buy them, and some are partially shelled but still have the shelly tail (which you would have to remove).

"deveined" means to take out the waste from the alimentary canal, the intestine.  It isn't really a vein.You need to peel off the thin muscular covering to open up the "vein" and rinse out the poop.

People generally don't want to eat shell or poop even when the poop isn't really harmful at all.

The 400 g value is not intended to be precise, so if you have a bit less after shelling and deveining, it should not make a difference.  Ideally, though, you would start with 400 g of already shelled and deveined prawn.  Not required, but that is the idea of the recipe.