It is likely a hyperbole or metaphor, but the context would be necessary to say for certain.
"It's raining flowers" could mean that flowers are falling from the sky like rain. On a windy day in spring, I stand under an apple tree. All the apple blossoms are falling down around me. I laugh and say to my friend, "Oh, look! It's raining flowers!"
It could also mean that so many flowers are appearing out of (seeming) nowhere in a short space of time that they are as abundant as raindrops in a rainstorm. If a secret admirer begins to send me many flowers several times a day so that I am overwhelmed by the continuous appearance of new flowers, I might say that it is raining flowers. There's a song where a woman sings that "it's raining men" to describe a situation where she discovers an abundance of available men. More men keep appearing out of nowhere the way that rain falls out of the sky. In both cases (the rain and the men), there's a lot of it, and we don't see where it comes from.