The rectangle is the internationally recognized symbol for a rear windshield. A front windshield's internationally recognized symbol looks sort of like a curved version of it.

The three curved lines is the international symbol for 'heat' or 'defog'. Combined with the international symbol for the rear windshield, it means rear defogger.

When you push the button, electricity flows through the defogger grid on your rear windshield. It's wiring connections are on the side usually via slide on connectors.