In the fairy tale, the Queen wants her son the Prince to marry a true Princess, so she devises a test. Only a true Princess would be able to feel a small pea in her bed, which would prevent her from sleeping. Even if twenty mattresses were piled upon the very small pea, a true Princess would still feel the pea and not be able to sleep.
So, woman after woman comes to the kingdom, and the Queen puts them up for the night. The next morning, the Queen asks the woman how she slept. Woman after woman says how comfortable the bed was (all those 20 mattresses!) and how well she slept.
Evidently, the Princess and the Pea test was not well-known throughout the land.
Then, one woman comes to the palace and is offered a night's stay. The next morning, as usual, the Queen asks the woman how she slept.
"Awful! If felt like there was this small thing stabbing me in my back. And no matter how much I searched the sheets and blankets, I couldn't find it. And the mattresses were so soft and comfy. I don't understand why I didn't sleep well!"
And now the Prince had himself a wife.
It's not quite the same thing with writing and reading, but it's close. See, readers must not be true Princesses, because they can't feel the pea (your true message) under all those mattresses. Every time you the writer add fluff on top of the pea, you hide that pea from the reader. It's too much for the reader to have to figure out what that pea is and even where.
So, remove the fluff - the mattresses - and the pea will be revealed.
You can see this in action with this great example of editing. Much of those strikethroughs are getting rid of fluff, those pesky mattresses. What is left behind is the crucial pea.