What is an “internal slot” of an object in JavaScript?

Question

I\'ve tried to understand ECMAScript 2015 specification in one point: Internal Slots of Objects. But this section appeared very unclear to me, especially this sentence:

Answer 1

It's simply an artifice used to be able to describe precisely how the objects should behave.

They are not real members of the objects and even if in some implementation they are you are not allowed to access them with portable code.

In other words it's a way to write the specification that allows describing behavior with imperative code that is formally more precise that just using a wordy "natural-language" description of what the behavior should be.