MX Player Codec (ARMv6) is exactly what its name suggests: a codec needed to make MX Player work on devices with ARMv6 CPUs. If this is the case for your device, you need to download this application in addition to MX Player.
If your device doesn't have an ARMv6 CPU, this application is absolutely useless, so don't bother downloading it. Also note that MX Player will probably notify you if you need to download this application.
