Probability is the likelihood of an event happening (or not happening). Generally multiplication would be used when calculating the probability of event A and event B happening, while addition would be used to calculate the probability of event A or event B happening.
For example, the probability of rolling a die and getting a six is 1/6. The probability of rolling two dice and getting two sixes is 1/6 x 1/6, or 1/36.
But the probability of rolling a die and getting either a five or a six can be calculated as 1/6 + 1/6 or 1/3.