This is the 6th most frequent Spanish word.
un
"Un" is the Spanish indefinite article meaning "a" or "an."
Used as an indefinite article to introduce a singular masculine noun.
Compré un libro interesante.
I bought an interesting book.
Used as a numeral adjective implying an unspecified day.
Un día descubrirás tu camino.
One day you will discover your path.
Used as part of an adverbial phrase to signify a small amount.