Quorn is a micro-protein made from edible fungus,
according to Wikipedia. It's mainly used as a meat substitute - I use it partly because it lets my hubby pretend there's meat in the pie, but also because I'm weird and like the texture (it can look like meat, but the texture is very different. And thankfully nothing like tofu, which apart from in Cauldron Foods sausages tends to make me heave). It's also very good at absorbing flavours. And it's high protein, low calory.
It's not neccesary for the pie - it would work equally well with decent mushrooms or waxy (not floury) potatoes instead.