An Anti-Air describes attacks that are meant to be optimal in the function of hitting airborne opponents in Street Fighter.
In terms of Hitboxes, anti-air attacks usually extend straight up or at an upward angle. These moves are generally used to attack an airborne opponent; whose likely jumping-in toward them. A few characters' normals have an upward hitbox that primarily serve as an anti-air attack, such as Ibuki's Agemen. Many characters are also equipped with special moves that are their main anti-air tools, such as Cammy's Cannon Spike or the iconic Shoryuken.
Anti-airs are usually defined in power. Jump-in attacks can be so dominating by some characters such as Pre USFIV Sakura who was infamous of having a disappearing hurtbox underneath of character model (crotch down) when using her Jumping hard punch. This let her bully a high amount of the cast who depend on 1-hit anti-air normals and Unique Attacks. Other characters such as Vega have strong jump in attacks that weaker anti airs can not defeat. Usually Anti Airs are balanced by character archetype. Dhalsim is known for having good anti air normals that cover different angles as his type of character depend on a zoning poking playstyle.
It is advised that players learn their character's primary anti-air tools to not only inflict solid damage, but to also take out their opponent's aerial options. Discouraging jumping takes away a tactic that allows the player to come on at positive Frame Data on both hit and block and keeps the opponent having to find new ways to approach. Even characters such as Sagat who have a powerful anti air in Tiger Uppercut should learn to anti-air normal in the case he is battling a character who can delay or change their trajectory in the air, as these attacks leave the user vulnerable to punish.