CPU shim
A CPU shim (also called CPU spacer) is a shim used between the CPU and the heatsink in a computer. Shims make it easier and less risky to mount a heatsink on the processor because it stabilize the heatsink which prevents you from accidently crack/damaging the fragile CPU core on the processor. They help distribute weight evenly over the core surface.
CPU's are one of the more costly components of a computer, so a CPU shim is a cheap insurance.
They are usually made of thin and completely flat aluminum or copper. Copper have a good heat dissipation capacity but are electrical conductive. Aluminum shims are often anodized which besides making them look better also make them non-conductive which is good. Anodized shims are often dyed with a color, e.g. purple. It is very important that they have the proper thickness because if it is too thick, then the heatsink wont make contact with the core.
They should be 100% non-conductive to prevent short circuit.
Most are CNC manufactured. The pattern is often laser cutted. Cheaper ones may be pressed or stamped which could make them less accurate.
CPU shims are not common in OEM computers, but sometimes used by computer hardware enthusiast who may use heavier heatsinks because they wish to have a more cool or silent system or perhaps to overclock the CPU for better performance. A heavy heatsink puts more pressure on the processor. Shims are very useful for people who often change CPU, heatsink or cooling solutions or use a heatsink that is heavier than the CPU manufacturers recommended weight.
They are cheap and can usually be bought for less than 10$ USD.