MX Player Codec (ARMv7 NEON) is exactly what it says on the tin: a codec needed to make MX Player work in devices that have ARMv7 NEON CPUs. If that's your case, you'll need to download this app in addition to the MX Player.
If your device does not have a ARMv7 NEON CPU, this application is completely useless, and therefore, you should not download it. It's important to note that it's usually the MX Player itself that asks for the codec in the case it needs it.
