Fiddle-faddle is silly, insignificant nonsense. Fiddle-faddle doesn't amount to much.
Fiddle-faddle usually refers to nonsense that is particularly insubstantial: trivial stuff that means little. If you're trying to discuss something serious, and someone makes an irrelevant point, you could say "That's fiddle-faddle!" This is a reduplicative word — like hocus pocus and higgledy-piggledy — which means most of the word repeats. Lots of reduplicative words mean nonsense, such as mumbo-jumbo and jibber-jabber.