The Midgar Zolom save glitch refers to a trick in Final Fantasy VII that allows the player to travel across the marshes without encountering the Midgar Zolom. Once on the marsh, the Midgar Zolom's shadow makes its way towards the player and engages them in a fight when it reaches them, making it difficult to cross without a Chocobo. However, if one saves the game before it reaches the player, then exit and reload the game, the Zolom will be on the other side of the marsh, giving the player time to make it across.
This is probably intentional, as otherwise it is possible to get oneself into an unwinnable situation several hours into the game. Supposing that the Zolom works as "it should", if a player made the unfortunate mistake of saving on their only save slot right next to Zolom, it's very likely game over, since the Zolom is quite powerful when first encountered. Furthermore, saving the Zolom's location would bloat the save files (especially since it is an extended object, and not just a point) - but, in its current state, the game probably spawns the Zolom at a random location far from the place the characters spawn.