What do you call the basic meaning or main idea about life that an author wants reader to understand?

The basic meaning or main idea about life that an author wants the reader to understand is called the theme of a work.

Here are some other terms that are sometimes used to describe the same concept:

* Central idea

* Underlying message

* Moral of the story (especially in shorter works like fables or parables)

* Universal truth

While a theme can be explicitly stated in a work, it's more often implied through the characters, plot, and setting.