Exposition
The background information that sets up characters, setting, and situation.
Exposition gives readers the context needed to follow a story, including who the characters are, where they are, and what circumstances they face. Writers usually place it early but can weave it in throughout. Skillful exposition informs without stalling the narrative momentum.
Example
The opening exposition establishes Della and Jim’s poverty and devotion, setting up the sacrifices that drive the story’s ironic ending.
The Gift of the Magi · O. Henry