Barrier Skip is 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. The easiest and most consistent setup is detailed in the video below and requires having a shield.
This is another setup that can be used to achieve the angle required for Barrier Skip.
This is another setup that can be used to achieve the angle required for Barrier Skip. For this setup, you want the drop shadow of the R button to kiss the darker part of the pole on the right side of the screen while the grapple hook bobbing animation bobs to the right.
For this setup you want to have between 10 and 19 bombs. Equip Bombs to your X button. After performing knockback cancel go into the far right corner. now you want to find the angle by repetitively going into first person cam and hitting target to check your angle. when the camera settles after hitting target the 1 on the amount of bombs should line up with the edge of a dark and a light texture on the screen. Once you have this you have the correct angle for barrier skip.
After canceling knockback you want to go into the far right corner. Then pull out telescope and zoom all the way in. The slowly zoom out until you get the second most zoomed in setting. Now line up the barrier (the dark texture) with the left side of the telescope view. Then if you charge an item slide and hit up and then target you should have the correct angle to clip through.
This setup does not require manually acquiring the angle. Instead it use various inputs to get the exact same angle each time. This is also the slowest setup by far.