Answer:
The word or is used in probability to denote that you are computing the chance that either event A or event B occurred; both events do not have to occur. When you employ the word and, you're implying that both A and B must occur.
In Conclusion:
If either one or both of them occur, the condition is fulfilled, and the probabilities are added.
The condition is satisfied if and only if both of them occur, and the probabilities are increased.