The left side is the cumulative probability that it's been kept "so far". The right side is the probability it will be kept, rather than replaced by the new el

The left side is the cumulative probability that it's been kept "so far". The right side is the probability it will be kept, rather than replaced by the new element (probability that the new element will not replace this one). Multiply them together and you get the overall probability that the element remains after the new element has been processed.