I think I've heard it in the context of speed-running Pokémon and other games. I think it means luck. It can be found around Arqade, but no one has asked what it means (as its own question).
RNG stands for random number generator.
It is often used in speedruns, to say that you had luck (like "I got lucky with the RNG") or that a section can be quite random ("The RNG in this section can be a pain").
You might also hear rngesus ("pray to rngesus that it works") - basically praying for good luck.