RNG means a random number generator. These are computers. The spinning reels are only used for effects. In a physical slot machine, it has drums or cylinders where the symbols are pasted. Once they spin, it is nature that stops the drums. All the results of the physical rotating drums are random. In a computer, the RNG is software imitating the randomness of nature. Once you hit SPIN, the RNG randomly stops the reels from "spinning". What it does on the background is it selects a random number per slot to appear. So if the cherry is number 7, and the RNG picked 7, the picture of cherry will show on a slot. The RNG selects a random number per slot.