it means that good (or bad) things do not just happen a few at a time, but in large numbers all at once. eg. Fred: I can't believe this. This morning I had a flat tire. When I went to the garage to get the tire patched, I discovered I didn't have any money, and I couldn't even charge it because my credit card's expired. Jane: It never rains but it pours. or something that you say which means that when one bad thing happens, a lot of other bad things also happen, making the situation even worse. eg. " First of all it was the car breaking down, then the fire in the kitchen and now Mike's accident. It never rains but it pours!" source: http://idioms.thefreedictionary.com/It+never+rains+but+it+pours :)

The actual saying is "When it rains, it pours," and it means when something you're expecting or looking forward to never happens or doesn't happen much, suddenly you get it a lot (it happens a lot suddenly)...