Idioms, idioms

how do you percieve this sentence?
what does is mean:
... .if I decided to be an actor, I’d probably be a great hit in the movies, she'd throw cold water on it.

can you, pls, rewrite it in plain words?

thanks very much

Throw cold water on smth — decrease (cool down) one's optimism about _the_ something.

I didn't know that idiom — it's just intuitive!