An RNG is the software that decides the result of every spin on an online or modern machine slot. It generates numbers continuously, and the value taken at the moment of your spin is mapped to the reel positions you see.
Each spin is independent. The RNG has no memory of earlier results, which is why there is no such thing as a slot being "hot", "cold" or "due". The animation of spinning reels is presentation: the result is already fixed when the reels start moving.
The RNG does not decide how generous a game is. That comes from the maths model: how numbers are mapped to symbols, which sets the RTP and volatility. Mapping several numbers to one symbol position is the virtual reel idea patented by Inge Telnaes in 1984.
In Great Britain RNGs must be tested by an approved test house under the Gambling Commission's technical standards. Most use a PRNG, an algorithm rather than a physical source of randomness.