A and B are two events linked together. If B happens, then A is more likely to happen because rain makes the roads slick. Also, it's hard to see if it rains very hard. There are probably other factors I'm not considering.
So again, to summarize, the two events of "it rains" and "there's an accident" are dependent. One event influences the other so they are linked together in terms of probabilities.
Note: Keep in mind that event A does NOT alter the probability of event B. If you have an accident, then it doesn't make the chance of rain go up or down. This is a one-way type of dependence.
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