Storage is a trick that gives Link a significantly smaller hitbox and hence allows Link to roll up very steep slopes and walls and even to clip through some slopes if the gradient of the slope is correct. Unfortunately, due to requiring Map Glitch and a certain type of door, the glitch is quite limited and currently has no use in speedruns.
Perform Map Glitch and then enter a door with a floor behind it for Link to stand on (using a door with no floor behind it will result in a softlock). Once inside the door, have Link hold any item (sword, Boomerang, etc.), then leave through the door. Link will now have a very small hitbox and strange collision that can ascend very steep slopes.
This glitch's usefulness is limited because Map Glitch disables all load zones, meaning Link cannot leave the area with Storage. Furthermore, there are not many doors in the game, and not all of them work for Storage.
An example of a potential use is Faron Escape without the Boomerang. Storage can be performed using the door to Coro's house, and then it is possible to get up onto the wall and around the Midna trigger to the loading zone. However, Map Glitch cannot be disabled and the savewarp trigger at this exit doesn't appear to work until after Forest Temple is beaten, so it is not possible to reach Hyrule Field with this method.
Original video by Zeldagamern01: