Anything described as antiaircraft defends a position on the ground from an air force. An antiaircraft gun shoots at attacking plans.
This is a military term with a very specific meaning. Like all anti words, this is in opposition to something: in this case, fighter planes and other attacks from above. All antiaircraft devices and strategies involve protecting forces on the ground from forces in the sky. Since planes have tremendous advantages in dropping bombs and firing at the ground, antiaircraft forces are important.