Interiority or "internals" is a fancy way to describe the reader's view into your characters' thoughts, feelings, and inner lives. Some genres require a lot of this (memoir), some much less (thrillers). Interiority is what makes a character real to the reader. Skilled writers reveal interiority in several ways. It's important to know what your genre requires and how to plant and build the interior lives, without having them slow the momentum of the story.