A simple example is: in coin toss, assuming that the coin is even and tossed twice, we know that the probability of a coin is 1/2 without giving conditions, but we give a condition (a coin is known to be upside down), so the probability of asking a coin to be upside down is called conditional probability, which is 2/3.
The principle of step-by-step multiplication is a mixture of multiplication principle and addition principle.