An air block system allows you to block while in midair.
It is implemented in only some games, and as such, the details on it vary from title to title.
When an air block occurs, the blocker is usually pushed backward some bit, altering the endpoint of their jump.
In many games, an air block can only be used during vertical and backwards jumps.
In the KOF series, it existed from KOF ’96 to KOF ’98; in ’99, it was removed.