This is the 8th most frequent Catalan word.
una
The Catalan word "una" means "a" or "one" (feminine).
In this sentence, "una" is used as an indefinite article to denote a single instance of a noun (a red apple).
Tinc una poma vermella.
I have a red apple.
Here, "una" acts as a numeral within a temporospatial expression, indicating a single occurrence or instance (a time).
Vaig veure una vegada a la montanya.
I saw a time in the mountains.
In this context, "una" is an indefinite article used with an abstract noun to express an undefined but singular instance (a great joy).