What is this thing sitting in slot 2 of my new DS lite?

It's simply there to plug the port (protect it from dust) when you don't have a GBA game in it. There's no real purpose to it, it just makes the device look more complete, rather than having a huge hole on the bottom.

The Cartridge Slot Cover is a replacement for the one included with your Nintendo DS Lite. The Cartridge Slot Cover is a removable cover that keeps the Game Boy Advance cartridge slot clear from dust and debris when it's not in use. - Nintendo Online Store

The contacts are to keep the card slot contact bus clean, burnished, and free from plastic residues.

Plastic wheels on model trains can leave a buildup on the track which functions as an insulator and is hard to remove. If the placeholder card in the DS Lite were all plastic, a similar buildup could accumulate on the DS's electrical contacts and interfere with it reading real cards.

If the slot filler card had only phenolic circuit board material, however, it would cause wear to the card slot bus, and would not mate smoothly with the copper card slot fingers. Having a card in the slot rather than leaving it empty also keeps the contacts from moisture and corrosives in the atmosphere.