The theory states, roughly speaking, that at any one instant in time reality has an infinite choice of crossroads to explore. At that instant Brian could choose to stop to look at a butterfly, or not. Either of those options is included in infinite permutations of arrangement of all the other bits in the universe, including the explicable bits, the inexplicable bits, and the irretrievably wrong bits.
Let’s say that reality, being rapaciously inquisitive, decides to wander down all of these roads and so splits itself into an infinite number of clones, so that each clone can take a separate road.
Each clone takes a quantum step down it’s respective road and is then instantly offered another infinite set of crossroads, including one where Brian ignores the butterfly and starts humming something by Abba, another where Brian decides to eat the butterfly, and another where Brian and the butterfly spontaneously swap existences in a rare display of
‘transposivity’ : a shy physical phenomenon which will repeat itself, by definition of the Multiverse, infinitely many times with all the possible pairings of creatures in the entirety of possible realities, including you and your postman in the reality where he has see-through skin.
And so on and so on until at some point, at some juncture in the Multiverse, Brian and a carrot will find themselves looking up at a doctor.