In Street Fighter games many attacks, both Unique Attacks and Special Attacks and even occasionally Target Combos can only use their attacks when the performer of the attack is jumping. Height restrictions are a certain point in the jump that a character can perform their attack. Trying to use the attack before the character reaches that point will either have the character perform another action or the game will wait for the character to reach the point of activation then continue as usual.
As Street Fighter games try to balance characters, Height Restrictions are used on a per character basis to keep a certain character from being overpowered. In Street Fighter IV since Rufus's Falcon Kick is so central to his gameplay Falcon Kick does not have a height restriction to help ensure he can use it to punish low attacks, Crouch Techs and grab attempts, as a tradeoff Falcon Kick does not have a very dominating hitbox, meaning it can barely trade with other attacks, let alone beat other hitboxes. Cammy's EX Cannon Spike doesn't have a height restriction either, while her regular version does. The tradeoff here is that Cammy needs to use Meter for her attack.
For players who use characters that can use aerial only attacks, it is important for those players to learn the height restriction of said attacks. Characters like Yun have very powerful dive kicks but has to deal with a restriction. For characters without height restrictions, it is important for players to learn a motion to activate as instantly as possible, some instant attacks can benefit from learning the Tiger Knee Motion.