From OP:

If you have some good (or bad) examples of over/under explaining authors
You'll remember The Firm, starring Tom Cruise as Mitch McDeere?

I read the book by John Grisham.
Early in the story, Mitch wants to photocopy some papers. But this is a law firm and security is important. Mitch had to go through a long series of security steps to get the copier to work. Grisham used up about two pages describing all these steps in infinite detail.
I thought, "Wow. This must be very important to the story."

It was never mentioned again! Apparently it was something that interested Grisham, but it had no point.

That was overexplaining a procedure. More commonly, the issue is how much to describe settings.
In To Kill A Mockingbird, Harper Lee spends much time describing the small Southern town in which the nine year old girl Scout lived. This was necessary because a small Southern town in the 1930s would be unknown to many readers, such as people in England and even Americans in Oregon or New York ( the book was published in 1960 ), but more importantly the whole point of the book was the environment's influence on Scout.

Sometimes the surrounding environment is not important. I just read Pride and Prejudice, and am now reading Sense and Sensibility. Jane Austen doesn't spend much time describing the countryside. Family dynamics are more important in influencing the main characters.

Sometimes the environment is unimportant. Let's say a main character spends time in Boston, New York, and London. Is it necessary to describe these places? Everybody already knows a lot about them. And, unless the main character is dragging his family along with him wherever he goes, even family dynamics aren't important, except in the sense that the main character is what the family made him when he was young ( but that's all over with! no need to make more than brief references to that )