I'm quite appalled at seeing people have such discomfort in having to answer a simple question regarding sexuality. Someone mentioned people not having the "courage" for the same, which quite stuns me. I think all of them avoided answering at the presumption that this question is a joke. Maybe this is.
The phrase which you asked about is vulgar. I'm not sure if this is a site where you can get your questions having to do with sexuality solved. This site is not specifically meant for adults. So vulgar phrases should at best be avoided.
I don't intend to question the validity of your ignorance. But if you don't understand what that phrase means, then either you're not mature enough to be understanding what it does mean, or you are deficit in practicality - good or bad.
P***y means the female genital. Raining refers to its ejaculation when excited sexually.
Now if you have questions regarding how that happens or stuff, you should rather ask the internet, or just another forum (please don't).
If you're not at the proper age for knowing or dealing with this information, I urge you to deviate. If this was a joke, congratulations you had a good laugh, I hope it makes you the happiest person on earth. And if you're just less knowledgeable about colloquial English, it's not a problem. These aren't words you're supposed to know in the first place.
Edit:
I saw you were active on the mathematics space. Mathematicians have a pure heart. I trust your genuinity.