Whats the Meaning of hey diddle diddle?

"The cat and the fiddle, the cow jumped over the moon." It's an 1882 nursery rhyme by Randolph Caldecott, unless you happen to be referring to a modern song I'm unaware of.

Edit:> Okay, I've just noticed this filed under Pittsburgh Steelers and have to assume I've made a slight error in my answer, perhaps.