As in real life, it is just the ways in which the characters of a story interact with each other.
Explanation:
It's not a fancy term - but unlike real life, how it is developed in a story can be critical to the plot development and the reader's enjoyment.
You can learn how your personality projects to others, how fast or slow you talk, if you have a high pitched voice or low, what someone else says to ake you laugh the most, your pain tolerance, and how you treat new people in your life.
