The Exploration Glitch (EG) is a very powerful glitch where Link walks on the wrong layer of the map.
Using EG in the underworld is banned in all categories of the ALTTPR Racing Ruleset, because it allows you to walk to the Triforce and skip all the requirements of the game. Using EG in the overworld is allowed as an overworld glitch, but ends up not being useful in most glitched modes. Overworld EG is mostly useful for leaving Hyrule Castle Escape (NL) in Standard Start No Logic.
EG is more completely described on the vanilla ALttP wiki.