"It's coming down in buckets" - An idiom referring to heavy rain, precipitation. Probably originally a contraction of "It's coming down in bucketfuls".
I always find idioms to be fun. I've been called a pedant, and when I am a pedant I try to be a thorough one. It'd be pretty awful if rain ever did come down in buckets, but (for example) I could imagine this expression being used literally in a skyscraper construction site, if by accident supplies were lifted too high originally and had to be lowered again.