Kaana as Emphasis
The word kaana is the past-tense masculine singular third-person (he) form of the verb to-be. So you can translate it as “he was”. (And the khabr takes the same rules as with any other verb.)
So what does it mean when Allah (سبحانه وتعالى) uses kaana to describe Himself? For example, in surah Nisaa, He says:
إِنَّ اللّهَ كَانَ غَفُوراً رَّحِيماً Translation: And seek the Forgiveness of Allah; surely, Allah is Oft-forgiving, Most Merciful [Surah Nisaa, 4:106]