A High Attack is an attack in the Tekken series that is aimed towards the head and/or upper chest area, whether it be a kick or punch. Most high attacks can be avoided by a regular standing Guard, or by crouching. Most Throws are also High Attacks.
High attacks are often very fast and safe moves. Jabs, for example, are highs. However, highs can still be crushed by certain moves or stances, meaning they can be punished despite how safe they are in other aspects. This makes high attacks best for pressure, and, similar to mids, makes high attacks good for lower damage pokes.
In Tekken 2, there was a unique variation known as Special Highs where an attack, despite being officially a Mid, could be treated as a high against a small character like Jun Kazama or Wang Jinrei, and vice versa. This issue was fixed in Tekken 3 where the standing hitbox of smaller characters was adjusted, but made a surprise return in Tekken 4, where a oversight caused King II's Jaguar Straight to miss against crouching small characters like Xiaoyu though this was immediately fixed in the next game.