It’s a fairly common trope in science fiction, and might even be science fact. The idea is that realities split from every decision point, some we’re aware of and some - due to quantum fluctuations - we’re not. Indeed, it might only be the quantum weirdness that’s valid and human decisions are merely emergent phenomena.

If you take a look at any quantum experiment, you get things like particles interfering with themselves and apparently appearing in many places at once. Tissue thin neighbouring universes along some probability axis interfering with each other, one for each possible position of a particle, would explain what’s known as the “many worlds interpretation”. The MWI doesn’t talk about a probability axis though. That’s the fictional part until proven otherwise.

It would still be a dimension even if things were more discrete though. Like, separately identifiable parallel universes where no intermediates exist. Hopping from one to the other could still be interpreted as moving within some extra dimension, and there’s nothing really stopping us from calling that probability.