Barrier Skip a sequence break in which the player can clip past the barrier encompassing Hyrule Castle. This skips the need to activate all the flags that are required to break the barrier and gives the player access to Ganon's Tower early.
There are two parts to being able to skip the barrier. The first is the visible knockback barrier which will constantly attempt to knock link back to the castle when you're inside it. To cancel this effect, you must get knocked down by it once, and then while inside it, pull out a cutscene item (usually the Wind Waker) on the last frame of Link's invincible flashing animation. If done correctly, the barrier will oscillate but Link will remain undamaged and in place. The second part of this skip requires executing an Item Slide with the correct velocity to clip through the barrier. It's possible to get the angle required for this velocity by first getting into the left corner between the bridge railing and the barrier. From here, go into first person mode, tilt the camera all the way up, and line up to a specific set of pixels when the castle's Triforce emblem is on the top left of the screen (This visual cue is explained in detail in the video below). Doing this should put Link at an angle between 48319 and 48304 (the angles that are known to work for this setup). If you're within this range of angles, you can then begin an Item Slide forward for about 1.5 seconds, then quickly press and release Up on the left control stick (do not press forward if using the hookshot) and tap ZL (hold ZL if using the boomerang). From here, Link will begin to slide along the barrier until he hits a certain position, at which point he will clip through provided that the angle is within the range specified earlier and that Link has anywhere between 40-6000 units of speed. If Link shoots out of the barrier to the left when you press ZL, then you either did not hold Up for long enough or you pressed ZL to early after holding Up.
This is another setup that can be used to achieve the angle required for Barrier Skip.