"Probability" is the term used to describe the likelihood of something occurring. In this case, it is the likelihood of temperature dropping to freezing or below freezing - a "freeze event". Mathematically this is quantified through the ratio of the number of historic freeze events that have occurred on, before, or after a specified date to the total number of all scenarios (in this case freeze and non-freeze events) that have occurred on, before, or after the same date. If a probability is low, it means an event is not as likely to happen. If the probability is high, the event is more likely to happen.