On classically designed slot machines, each reel has a set number of positions that it can come to a stop upon. Each spot is called a “reel stop,” as the reel can stop in that position.
On physical reels there’s a physical limit to the number of positions that can be stopped simply because a reel can only be so big, and each space so small. Virtual reels (those that are created and generated by a computer) can theoretically be infinite, although mathematically there’s no need to take it beyond a certain size.
Virtual reels can get into the hundreds of spaces as necessary to accomplish the odds the game can acquire. Imagine a game with 5 reels, each with 64 places to stop, and an equal chance of stopping on each reel stop on each reel. That could offer 1 billion combinations of reel stops. Now, there aren’t that many symbols, so that doesn’t mean the top prize odds are that low, but it shows you how much control game designers can have over a game.
When the RNG is hit for each reel, it selects a number that maps back to a reel stop, and that’s where each reel stops. Then the pays are evaluated and you’re paid as a player if the symbols line up to a payable outcome based on the pay table.
Keep in mind that a reel stop accounts for a given location on the visible reel zone. While early slot machines had a single line across the middle, and the reel stop anchored there accordingly, nowadays the space or spaces above and below the reel also factor in, given many slot machines with a 3×3, 5×3, 5×4 or beyond, or even weird diagrams of reel spaces like Buffalo Diamond, can have an impact on what is visible and active on a payline.
Another thing to remember: Just because there’s a certain number of reel stops doesn’t mean they have an equal chance of stopping on each of the reel stops. Some games are designed with virtual reel mapping (especially those limited physical reel games) which can allow for some reel stops to land more frequently than others, allowing for more control over the frequency of various outcomes beyond just a basic odds of stopping on any given reel stop.