(covers information from several alternate timelines)
A temporal inversion fold is a type of parallel time stream in the spacetime matrix. The folds were visible as temporal anomalies intersecting the prime reality. Inside the folds, reality remained unaffected by changes in the timeline.
In an alternate timeline, in 2372, Ensign Harry Kim encountered an alien from a temporal inversion fold, who had taken the form of a Human named Cosimo. The alien told Harry that the creation of the alternate timeline was a result of Harry intersecting with one of his race's timestreams in a shuttlecraft. Cosimo then detailed to Harry how he could recreate the accident, thereby repairing the damage to the timeline. (VOY: "Non Sequitur")