I didn't actually know what this item was, but I searched the code on the packaging - 561185425 - and the Amazon result suggests that it is something called a telescopic oven rail. Here's the link to something similar:
https://www.amazon.co.uk/Rail-Telescopic-BEK435111M-561185424-925886402/dp/...
Still in packaging. I can't remember why I have it, but maybe it's left over from the installation of my oven. Collect from Llanishen.