RNG is a random number generator. It's a seemingly random sequence of numbers that games use for all kinds of elements that are supposed to be unpredictable to the player.
Since computers are actually unable to just spit out anything truly random they are usually actually based on things like the system clock (which changes incredibly quickly) or in other cases on the previous number + some fancy math that makes it so that we can't tell what the number is going to be.
Here's a hugely oversimplied example: Let's say the game wants to decide if the flower you watered yesterday is going to spawn a new one today.
It consults the RNG, which will return a calculated number between say, 1 and 100, and if the number happens to be above 85, it makes you a new flower, which means there is a 15% chance of you getting a new flower today. Since you don't know how it picked the number (or are unable to influence it), it will be random to you.
