Conditional probability is about calculating how lilely one thing is given knowledge about something else. For example, assumimg no prior knowledge, the probability that the ground is wet in your area might be 50%, but if you heard recent rainfall then the conditional probability that the ground is wet may be 99%. Conditional probabilities are usually denoted using a vertical bar "|", for example:
Prob( ground wet ) = 0.5
Prob( ground wet | rained earlier ) = 0.99
. Conditinal probabilities may be used in or calculated using Bayes Theorem.
Used on pages 42, 50