How am I meant to use these trays that came with my oven?
Unlike American ovens, some European ovens come with built-in baking trays.  These are designed to slide in and out using the oven rails, and take the place of sheet pans that you would put on top of a rack in an American oven.

They're actually quite convenient, except that there's only two of them.

For example, if you watch this Spanish cookie video, you'll see that Albert is using one of those pans to bake cookies.