Conditional probability is the likelihood of an event occurring given that another event has already occurred. It is denoted as P(A|B), which reads as "the probability of event A occurring given that event B has occurred." This concept helps in understanding how the occurrence of one event can influence the probability of another. For example, consider a deck of cards. If we want to find the probability of drawing an Ace given that the card drawn is a Spade, we would focus only on the Spades in the deck. Since there is one Ace of Spades among the 13 Spades, the conditional probability would be 1/13.