RNG is an abbreviation that stands for Random Number Generator. The term is most often used in video games or casino games and it refers to a software algorithm that generates random outcomes.
There are two types of RNG generators — pseudo-random number generators and real random number generators.
The pseudo-random number generator uses a table of predefined constants and various mathematical formulas to produce a “forced” random result based on an algorithm. In other words, the result isn’t actually random but only appears to be such. However, the result is virtually impossible to predict and it still provides a sense of randomness.
When it comes to the real number generator, to generate such a number, a computer uses unpredictable events from the outside world. For example, your computer measures a physical phenomenon that occurred in the real world or takes a random event such as how many seconds have passed since you pressed a key on your keyboard. Since you are not predictable like machines or algorithms, a computer is able to create a truly random number by using that information.
An important thing to note is that, in order for a casino or video slot developer to get a license, they must pass rigorous tests to verify the fairness/RNG of their games. Therefore, if you are playing in a licensed casino that offers games from licensed software providers, you can rest assured that their games are fair.