اشتَرَيْتُ كِتَاباً I bought a book.
مَرَرْتُ بِرَجُلٍ I passed by a man
It is unknown which book I bought and which man I passed by.
But that's only the tip of the iceberg!
When one studies more advanced books of Grammar and delves into the science of Rhetoric then things get more complicated, but interesting at the same time.Indefiniteness, as Explained by the Scholars of Rhetoric, can have many possible meanings and the only limit is ones own imagination.
One of the ways which Ancient Arabs used it was to magnify something. In other words, something is so great that one is unable to define it by making it definite. It is as if, it has no limit. With this meaning we could translate the following sentence as
مَرَرْتُ بِرَجُلٍ I passed by a GREAT man
This meaning would be applicable if the context mentioned the great qualities this man.
Another example is the word كِتَاباً in the ayah mentioned in the following short video lesson. If you notice, Indefiniteness is used to magnify the Quran.
I have explained this topic along with many others, giving examples from both inside and outside the Quran, in the following Course: