A brake pad shim, also known as a brake pad backing plate or brake pad insulator, is a thin piece of metal that is positioned between the brake pad and the brake caliper. Its primary function is to dampen vibrations and reduce noise generated during braking. Shims are commonly made of materials such as rubber, steel, or a composite material. 

When you apply the brakes, the brake pads press against the brake rotor to generate friction and slow down or stop the vehicle. During this process, vibrations and noise can occur, leading to brake squeal or other undesirable sounds. The brake pad shim helps minimize these vibrations by acting as a buffer between the brake pad and the caliper. 

In addition to reducing noise and vibrations, automotive brake pad shims also serve other purposes, such as preventing heat transfer between the pad and caliper, improving the overall performance and longevity of the brake system. 

Shims are often included as part of brake pad replacement kits, and they are designed to be compatible with specific brake systems. They play a crucial role in ensuring a quiet and efficient braking experience while also contributing to the overall safety and functionality of the braking system.