High, low, and mid are properties of moves. Knowing this property of the move is important when blocking. In general, high attacks must be blocked while standing, low moves must be blocked while crouching, and mid moves may be blocked either standing or crouching.
Mid attacks make up the majority of ground-based attacks in most fighting games and can be blocked while standing or crouching.
High attacks are typically aerial attacks and must be blocked while standing. However, moves known as Overheads hit high even though they are grounded. Overheads are typically command normals.
Low attacks are typically moves executed while crouching and must be blocked while crouching. Some moves, typically known as Sweepers, hit low even though it is executed while standing. These moves are typically command normals.
Since high attacks must be blocked high and low attacks must be blocked low, combining both moves in an offensive against an opponent keeps them guessing: do they block low or high? This concept is known as a high-low mixup. Experienced players often utilize both to catch opponents off-guard. Examples of this include using overheads while your opponent is blocking low, which will break through their guard and will allow you to follow up with further attacks.
However, attempting to mix up can be telegraphed. In Street Fighter, Ryu's Collarbone Breaker, an overhead command normal, is slow to start; a perceptive opponent will know to begin blocking high when he sees you using it.