So... Earthing the reader is a term I've taken to using when thinking about when it comes to reader perceptions.
Broadly speaking, it works like this: There's an aspect or thing that has an impact on the story or character. Say... a gun that kills an intruder. Easy, right?
Well... no. Because if you want to keep your readers happy and the tension going, two things need to happen.
1) The reader must be shown that there is in fact a gun in the house.
2) The reader must be on the edge of his seat,
biting his fingernails because of the fact that the intruder broke in.
Now for things to be made even better, the gun and intruder can be earthed further. For example by showing the reader that the gun in fact does not work and later revealing that the intruder is there to kill the main character. Drama abounds, right?
All because the reader was earthed right. If they aren't gently lulled into the story with wisely inserted but vital information, the climactic scene will not have an impact on the reader except apathy. Or worse: disbelief.
Earthing the reader isn't just about the storyline, though. It's in every scene. If your characters are eating, make sure the reader knows this before the character puts her plate aside. Every single thing that the character interacts with has to be set up, otherwise it looks like he or she is conjuring it out of thin air.
It also extends to character motivations. Characters shouldn't be doing things without reasons, so you should be earthing the reader by either showing their motivations in small ways before something big happens. Conversely, a huge, seemingly incongruous reaction can be used to set up an important revelation of a character's motivation.
All of the above have to be done with skill and subtlety. Make it too obvious and the reader won't like it either, finicky creatures that they are...
Look Out for These:
CP and Beta reader reactions are a major indication of problems in this area. Look out for these (or similar) reactions:
1) "I don't like the Deux ex Machina in the ending."
2) "Where did this come from?"
3) "Why is the character reacting like this?"
What do you look for when earthing a reader? Do you ever pay attention to it while editing?