This is the 3682nd most frequent Catalan word.
un
The Catalan word "un" means "a" or "one" in English.
Here, 'un' is used as an indefinite article for a singular masculine noun 'llibre'.
Ell va comprar un llibre nou.
He bought a new book.
In this case, 'un' is used as an indefinite article for 'avió', which is a singular masculine noun.
En alguna ocasió, vaig veure un avió volar baix.
On some occasion, I saw a plane flying low.
Here, 'un' is used as a numeral meaning 'one', referring to a singular item to be distributed.