This idiom means that when bad luck or misfortune happens, it often comes in large amounts or all at once. It describes a situation where troubles pile up, one after another.
Here are some examples of how it's used:
* "I lost my job, then my car broke down, and now my dog is sick. It never rains but it pours!"
* "First, they missed the deadline. Then, they lost the client. It never rains but it pours!"
Let me know if you had something else in mind!