Angle Skewing refers to the act of making Link face in any direction that is outside of the eight he is normally locked to.
This is the most common way of getting a useful angle skew. By jumping in the air and tapping any direction before Link lands, the angle he will be facing when he lands will be skewed towards the direction you push. Common applications for this are specific Hookshot angles that let you pull Link towards a target that is otherwise impossible to hit or specific angles for you to shoot an arrow to hit a target that is outside your regular 45-degree range.
Placing Link at certain locations relative to a tile of soil and then digging it up will also allow him to skew his angle. This method is much harder to control and less useful than using the Roc's Feather method.
Once you've skewed Link's angle, moving again with the left stick will make Link face one of the 8 regular directions. To avoid this you can charge and hold a spin attack, and then move around with the left stick to keep your skewed angle.