This is an aluminum heatsink kit used for cooling the chips on Raspberry Pi 3B and 3B+. The kit comes with 2 small plates and 1 large plate.
This heat sink set is compatible with Raspberry Pi 3B and 3B+ only. If you are looking for heat sinks compatible with Raspberry Pi 4 (1GB, 2GB,4GB), please check $0.99 Heat Sink Kit for Raspberry Pi 4B - Silver Aluminum
Update for the re_computer case