This is the 36th most frequent Turkish word.
bir
"Bir" means "one" or "a/an" in English.
'Bir' denotes 'one' as in expressing a specific day, 'one day'.
Bir gün bu hayalim gerçek olacak.
One day my dream will come true.
'Bir' signifies 'a' or 'an' as an indefinite article referring to the noun 'kedi' (cat).
Bahçede bir kedi gördüm.
I saw a cat in the garden.
'Bir' emphasizes the extraordinariness of 'harika' (magnificence), akin to saying 'what a...' in English.