Imagine these starts to a novel:
1. A character opens a letter - they've inherited a million pounds! Now they can go on the holiday of their dreams.
2. A character is having breakfast when they hear the letterbox rattles. They collect the post - a few bills and a letter. It contains the information that they've inherited a million pounds! Now they can go on the holiday of their dreams.
3. A character is talking to their family about their forthcoming holiday trip to a caravan in Norfolk. They've worked over-time and saved every penny to afford the two weeks and it means everything to them. One of the children is rubbishing the holiday - all their friends go abroad. The character defends the holiday: it will be miles better than abroad will be, lying on a pool by a lounger is awful etc. The letter box rattles and they collect the post, but they can't open the letters now, they've got to do the school run and then go to their job doing something that is poorly paid but gives great personal satisfaction. At lunch they remember the letter. It contains the information that they've inherited a million pounds! Now they can go on the holiday of their children's dreams and give up the badly paid job. But should they?
Normal World is the world of the main character before the story proper starts - in this case, the inciting incident (as the film world calls it) is the discovery that the character has inherited a million pounds.
Example 1 starts with no information about Normal World at all so the incident, while exciting, doesn't have much meaning. If the character was incredibly rich, for example, a million wouldn't have much impact at all.
Example 2 offers a little bit more information about Normal World - we'd be able to place the character by the sort of breakfast they were eating, the house they were living in, but we still don't see much meaning in the discovery.
Example 3 establishes Normal World. We learn a little bit about the character - hardworking, proud, parent etc. This means that the reader can appreciate that the discovery will have a huge impact on the character's life. It also suggests that the discovery might be negative or positive.
I learnt this lesson with Another Woman's Husband. I started with the discovery that will rock the main character's world, but my editor wanted me to establish Normal World because the reader needed to know what the character was risking if she strayed. What was Chapter 1 became Chapter 3 in the published book, and it's all the better for it.