The boot tray is a shallow box, the shape of the boot. It will protect the boot if you carry anything that could leak.
It also very useful if you carry anything that rolls around that could damage the boot sides or carpet. Well worth the money if you need it.
The boot mats come in various forms. Carpet, non-slip, all are fitted to the shape of the boot. The luggage net is very useful for stopping objects moving around in the boot.