Conditional Probability is the probability of an event occurring, given that another event has already occurred.
An example would be: What is the probability of a job applicant passing a panel interview, given that he has passed the initial interview with HR? Let's say 25% of all applicants are able to pass both the panel and HR interviews, and 42% pass only the HR interview. The probability of an applicant passing the panel interview given that he has already passed the HR interview, is equal to the probability of successfully passing both interviews (.25), divided by the probability of passing the HR interview (.42), which gives us .60, or 60%. This means that an applicant has a 60% chance of passing the panel interview, once he has passed the initial interview with HR.