noun, adverb
Set phrase used at the beginning of folk tales and stories, equivalent to 'once upon a time'. Often used adverbially.
昔々、あるところにおじいさんとおばあさんが住んでいました。
Once upon a time, there lived an old man and an old woman.
昔々
It's a story from long ago, but they say there was a big dragon in this village.
昔 is a general noun/adverb meaning 'old times' or 'long ago', while 昔々 is a set phrase specifically used to begin stories.
Reduplication of 昔 (むかし, 'long ago') for emphasis, creating a story-opening formula.