I know this word when I was in high school, from Coelho's The Alchemist. The Alchemist is one of my best favorite from Coelho's, along with Veronika Decides to Die. My first encounter with Paulo Coelho's book was incidental; I bought The Alchemist without knowing who Coelho was, and bought the book simply because the cover was orange, my colour.
The word means fate, destiny, whatever you want to call it. In The Alchemist, it was explained that you have to be born an Arab to understand it, but in English, its something like it is written. Its my favourite part from the book.
"Maktub." she said. "If I am really a part of your dream, you'll come back one day."
The whole idea about this word does not limited only to love/romance, but to dreams, rizq. The word also doesn't sound too grandiose, it sounds very simple yet it holds a very dear meaning.