RNG stands for Random Number Generator. It is used colloquially, in gaming, to refer to the probability of certain random events or the underlying implementation of them.
In its most personified form, the RNG transforms into RN Jesus to whom you pray for good video game luck.
In the case you weren't joking: RNG stands for Random number generator. It is an abbreviation commonly used in video games.
Also in cryptography, in things like CSPRNG (cryptographically secure pseudo-RNG)
That's exactly what it is. RNG = Random Number Generator.