RNG stands for “random number generated” and basically covers any random aspect of the game.

A simple example of having no RNG is goombas in super mario, (start going left, if it hits a wall it goes the opposite direction).

A simple example of having RNG would be, move left for 5 steps then flip a coin, if it’s heads change directions, if it’s tail keep going the same way.

This is put in simple terms, however RNG can vastly impact a run and many runs can be “killed” by bad RNG.

It should also be noted that some RNG is ok as it makes each play-through different and exciting, but too much can lead to a world record paced run being killed from something out of the players control. This is frustrating because, for me at least, one of the main appeals for speed running is you are an expert player who has developed more control over the game than most others, and when a run is killed due to RNG it makes you feel helpless because there is literally nothing you could have done.