What does RNG stands for? What is the meaning of RNG?

This means random number generator. The computer generates a random number from a big set of millions of numbers (normally) and displays the result. For example, an RNG game of European Roulette would have the numbers 0 through to 36 (so 37 numbers in all) in its register. When you press "Spin wheel" the RNG (random number generator) picks one of the numbers from the list.

Online, apart from live dealer casino table games, all of the slot machines and electronic games you play work using a Random Number Generator. The computer randomly picks the outcome of every game.

Some casinos use independent testing laboratories to check their RNG on their servers, to make sure it really is generating random numbers that are fair and reasonable over time. A fair RNG is essential for a fair computer game.