Hardware decoding of additional codecs on the Pi 3 and earlier models can be enabled by purchasing a licence that is locked to the CPU serial number of your Raspberry Pi.
On the Raspberry Pi 4, the hardware codecs for MPEG2 or VC1 are permanently disabled and cannot be enabled even with a licence key; on the Pi 4, thanks to its increased processing power compared to earlier models, MPEG2 and VC1 can be decoded in software via applications such as VLC. Therefore, a hardware codec licence key is not needed if you're using a Pi 4.
decode_MPG2 is a licence key to allow hardware MPEG-2 decoding, e.g. decode_MPG2=0x12345678.
decode_WVC1 is a licence key to allow hardware VC-1 decoding, e.g. decode_WVC1=0x12345678.
If you have multiple Raspberry Pis and you've bought a codec licence for each of them, you can list up to eight licence keys in a single config.txt, for example decode_MPG2=0x12345678,0xabcdabcd,0x87654321. This enables you to swap the same SD card between the different Pis without having to edit config.txt each time.
This article uses content from the eLinux wiki page RPiconfig, which is shared under the Creative Commons Attribution-ShareAlike 3.0 Unported license