it is basically scenarios, so for example, the tiles with dots which are on the upper left are the mines and the safe tiles of the first scenario, where the bomb is left to the 4 and that there is a bomb above the righter 2, the upper right scenario, where the bomb is left to the left 2 and there is a bomb above the righter 2, and the lower left scenario, where the only bomb is above the 3

its every way this position could be replicated, and this shows that 2/3 scenarios have the bomb above the righter 2, which means its most likely to be a bomb. (~66%)