This idiom means that when things go wrong, a lot of things go wrong at the same time.

I'll throw in an extra word to make it clearer: "It never just rains, but it pours". So bad luck is not simply one case of bad luck... it's bad luck, in spades.

The expression is an idiom, and idioms often cannot be understood literally (word for word). The picture that the expression paints / the message in the exprssion is that we feel a little bad if it rains, so the problems that we have in our lives are like the rain. People feel blue when it rains and they feel blue (sad/depressed/unhappy) when they have problems. But if there is too much rain or if we have too many problems it feels like the worst day of our lives. Now I will tell you a story: One morning it was raining hard and that made me sad because I had planned to have a picnic at noon. Then I decided to make some breakfast, but I burnt the food. I was hungry, so I ate the burnt food, and the hard, burnt bacon cut my the inside of my throat. That made me angry so I took the plates to the sink and the plates slipped out of my hands and broke on the floor. When I tried to sweep up the broken glass and porcelin I had to bend down and I felt a pain in my back. The I laid down on the couch to rest my back. I had a cigarette and fell asleep. The house caught on fire, but I woke up in time and escaped from the house. My car was parked in the garage attached to my house, so I drove it out onto the street to prevent it from burning. Unfortunately, as I was backing out of the driveway I was watching the house burn down and I failed to see the truck that was driving down my steet. Naturally the truck hit my car. I wasn't hurt, but the car was completely destroyed...... It didn't rain that day - it poured. Everything that could go wrong, did go wrong. A pouring rain is the same as a torrential downpour, like standing under a waterfall. When our life is so full of so many problems / so much rain we become incapacitated, unable to do anything, overwhelmed and rendered useless. On that day, when it poured, I was powerless.

Hey,Anna 我的一个中国的朋友告诉了我，It never rains but it pours 是 祸不单行 的意思。 Bad things not happen alone!

When things are bad, they are very, very bad. You never get a "bad" situation. You always get a very, very bad situation.