Drown warping is a technique that allows you to warp to the entrance of a map using the Goron mask (or Deku Mask, but that's hard) and deep enough water.
Basically what you want to do is stand next to deep enough water as Goron and tap the control stick towards the water. If done right, Goron Link will drown and you'll be taken to the coordinates at which you're taken to when the map is loaded.
The consistent setup found by Fox consists in doing two backflips as Goron, the second one should make you drown.
The first use of this glitch is grotto warping. Play Song of Double Time to get a night transition in Termina Field at the coordinates you want to be taken to, then get into a grotto that contains water. Perform Drown Warp and you'll eventually end up in either another unloaded grotto or you'll fall OOB. Here's a video of the glitch:
This is used in the 100% category. Basically, if you do the glitch at the end of Gyorg's Trial, you can get back to the start of the trial. This trick saves around a minute.