You have heard of the multiverse? The mathematical model in which all possible universes are equally real, giving rise to an n-dimensional manifold in which - say - the cup of coffee you are about to sip is, instead of being a little too cold, suddenly the perfect temperature, or is piping hot, or is a pink baboon made of knives, or is part of your hand, or is bigger than the entire Gillaxy1
The multi-deck is a little like the multi-verse, but somewhat less ambitious.
Imagine a room filled with infinite particles of random quant energy, zig zagging in random directions.... is what physicists say. But that's not what the Multi-Deck is like.
The Multi-Deck - the multi-deck within which our own universe so happens to exist, is more like this:
Imagine a room, cluttered, dusty, dim, filled with untidy stacks of VHS Tapes. Scattered here and there are televisions and VHS Video players, in various states of repair and decay. A muddled head woman stumbles around, trying to locate on which of N+1 ash trays she left her cigarette, and at random intervals she picks up a tape, pops out whatever's playing and puts on something else. Her pet cat -- Biscuit -- leaps from one stack of VHS tapes to another -- randomly landing on remote controls, hitting the fast forward button, or rewind, or instant replay, or sending the volume to maximum, or to mute, or somewhere in between. There is a window open and a stiff breeze that smells like approaching rain.
The Gillaxy is: 1. a galaxy, somewhat larger than this milky-way in which we live and also, 2. the Gillaxy is a dude called Gill, a regular dude, pretty chill guy. He doesn't mind being a galaxy, he just takes it in his stride. Last we heard from Gill, the Gillaxy, he was living in San Francisco, or thereabouts, with a gal named Apple Jones, or so he said. I'm not sure which San Francisco.↩