Flashback (analepsis)
A move back in the story's chronology to narrate an event prior to the narrative present.
Analepsis — Genette's term for the flashback — interrupts the narrative present to recount an earlier event: a memory, a childhood scene, a hidden episode. It answers a question the present has raised; that's its condition of efficiency. A flashback that illuminates nothing current is a disguised digression.
The classic traps: placing it too early (the reader doesn't yet care about the past of a character they barely know) and signaling it heavily. The most elegant transitions go through a sensory trigger — a smell, an object, a sound — that tips memory over the way it tips in life.
Example
Proust's madeleine: a present sensation opens an entire stretch of the past.