This is a metal sheet or tray, covered with a non-stick coating, similar to the lining of an oven.
It can be used for cooking a wide range of things, but mainly small items to be cooked together such as rock cakes or profiteroles.
If you intend to buy:
Buy one made of steel or aluminium, but most importantly with a non-stick coating, otherwise you will always need to line it with cooking parchment, or grease with butter, then dust with flour.