This is the 7th most frequent Italian word.
una
"Una" means "a" or "an" in English (feminine singular article).
Here, 'una' is used as the feminine singular indefinite article corresponding to 'an' in English.
Ho comprato una mela.
I bought an apple.
'Una' acts as the indefinite article for a singular feminine noun, indicating one unspecified shooting star.
Abbiamo visto una stella cadente.
We saw a shooting star.
In this idiomatic usage, 'una' serves as an indefinite article, setting up a traditional narrative opening for a fairy tale.