Something just happened. Did it work? How much? Fast enough? Where is everyone involved, now? Are you hurt? Tired?
No matter what system you're using, stuff happens in play. When that stuff involves doing something, then someone eventually describes it and says how it turns out. That's narration.
Don't mix it up with the ordinary idea of talking.
Also, some games are really vague about who gets to do this, relative to somebody carrying out a resolution mechanic, and some are really specific.
A type of Ephemera. What is said by a game participant to alter or add to the Shared Imaginary Space (sic -ll) during Resolution, especially Effect. How narration is distributed among participants varies widely, see Authority; to be fully accepted, narration requires Credibility.