Barrier Skip is the theory of getting to Ganon's Tower without breaking down the Hyrule Barrier. It is impossible with current tricks.
These images show the shape and size of the Barrier.
In this picture, the blue represents the edge of the knockback region. Everywhere inside can be freely walked around. Slightly behind that is the physical barrier. The physical barrier has no top. The red area is the knockback region. Touching the knockback region causes Link to be knocked back. This knockback can be temporarily canceled by pulling out the Wind Waker on the same frame that you stop blinking from getting damaged, or knocked back. The knockback gets re-enabled if you take damage or leave the knockback region.
This first picture is pretty self explanatory, but short. Theories:
1) Zombie Hover: The barrier extends so far up, that there is not enough horizontal space to get enough height to reach the top. Even if you got to the top, you would be knocked back because it is within the knockback zone.
2) Zombie Hover inside the damage field: In order to Zombie Hover, you need to take damage so you can have 0 hearts. This re-enables the knockback. To cancel it again, you need to stop blinking first, but you die before you are able to.
3) There is no way to clip through. The railing does not allow you to hang from it, so a clip is not possible.
4) Wind Waker Dive, then Zombie Hover: Due to the issue mentioned in #2, you can't have canceled the barrier and be dead at the same time.
5) Deku leaf: After the Wind Waker dive, you could use the deku leaf, but there is nothing to land on.
This is the most called upon trick when people talk about Barrier Skip. This does not work, because taking the damage re-enables the knockback region and pulling out the Wind Waker occurs before you stop blinking, so the knockback region is not canceled.
With cheats, the Barrier has been skipped. There are some oddities that come with it.
Entering the Puppet Ganon fight from Hyrule 1 causes the game to crash.
Entering the Puppet Ganon fight from Hyrule 2 or 3 gives you the Full Master Sword and the Mirror Shield.