Rotated Standard decks are preserved decklists from previous Standard environments whose cards are no longer collectively legal in current Standard. The tag is useful for archiving historical decks without implying they remain tournament legal.
Examples might preserve Mono-Black Midrange from Dominaria United-era Standard or older decks built around cards such as Embercleave, Teferi, Hero of Dominaria, or Thragtusk. They're all snapshots of what Standard looked like at a particular time.