For me it's one of the three layers.
First layer, it starts to wonder to the direction offered by real world. Wonder about song lyrics I'm listening too, or the colors of buildings, or something like that.
Second layer happens if you purposefully stray your thoughts away from a real world. Brain starts trying to remember whatever you wanted it to remember. For example, if I tell myself three times "don't forget to call mom", if I just sit idly, chances are my brain would surface "call mom". Sometimes it surfaces "remember you wanted to try arch 2 years ago". Also this is the layer that works badly if you tried or depressed, because it can start surfacing bad thoughts.
Third layer I call time skipper because it's just a basic time fast forwarding. I have no idea whether it's true meditation or controlled dissociation or something else, but thoughts slide of your brain, so there's nothing to remember afterwards. My friend said that maybe meditation is a controlled dissociation.
I prefer to say that I'm so dumb, I can just switch my brain off.