In English, a word sounds exactly the same whether it sits by itself in a dictionary, acts as the subject, or acts as the object. "Book".
In Arabic, nouns are "alive". They react to the words around them. Because of this, every noun must have a "neutral" or "default" resting state when nothing is bothering it.
Whenever you look up a word in the dictionary, it is in the Marfoo' state. When a word is the Subject (Mubtada) or the Predicate (Khabar) of a sentence, it remains in its peaceful, Marfoo' state.
Practice reading these words. They have not been attacked by any outside grammatical forces, so they rest in the Marfoo' state!
Marfoo', Mansoob, Majroor: you know how the last vowel tells the story.