Il pleut.
This translates literally as "It rains."
Here are some other ways to say it, depending on the context:
* Il fait de la pluie. (It's raining.) This is a more descriptive way of saying it.
* Il pleut des cordes. (It's raining cats and dogs.)
* Il pleut à verse. (It's pouring.)
The choice of which phrase to use depends on the intensity of the rain and the context of the conversation.