Multiplication rule

The probability that two events A and B occur is equal to the probability that a first event occurs, event B, times the conditional probability that event A, occurs, given that event B has occurred.

The probability that events A1, A2, up to An all occur is going to be the probability that the first event occurs times a product of conditional probabilities that the i-th event occurs, given that all of the previous events have already occurred. And we obtain a term of this kind for every event, Ai, after the first one, so this product ranges from 2 up to n.