The definition of the probability of event B given event A succeeds.
For example, if event A is the probability of picking a student with a red shirt and event B is the probability of picking a student with jeans, then the probability of B given A is the probability of picking a student with jeans out of all the students wearing with red shirts.
You must know the probability of B and A (the probability that both A and B are true) to use this equation.