An intentional gap in time used to advance a story without filler material. It is commonly used in film, novels, television and video games, to mark the passage without the intervening events being explained in any great detail, or merely in passing.
Timeskips can be as short as a few hours or bridge entire years' worth of happenings. It can also work in retrograde, though this is not as common.
Go watch a movie. Any time the screen wipes from a daytime scene to a nighttime scene in the same area counts as a timeskip.
TimeSkip is a term Aizen Espada 0 (DarkWorld) created in 2016 when he learned how to teleport with freeze lag most say he just borrowed the term from Dragon Ball Z.