ring around the moon?

I've heard it means rain but I don't know if that's right.

That ring, usually called a halo or nimbus, or sometimes an icebow, can appear around the sun, moon, or other bright light source, and is a result of reflection and refraction of light in the atmosphere. Specifically, the light is usually interacting with ice crystals in cold cloud-tops. These halos were sometimes used in antiquity as rudimentary methods of weather forecasting.