Super canceling is the act of canceling something abnormally with a super special move (or its equivalent). It was originally from Street Fighter EX.
It varies from game to game, but the basic version involves canceling the stun of a special move, activating a super special move (and depending on the game, perhaps into an even higher-ranking move).
Using this, you can put together otherwise impossible combos, racking up lots of damage. Also, this allows you to follow up a special move with a lot of recovery time.
This ability is generally restricted to only a few moves, such as in Street Fighter III.
In various series, there are systems with the same name, but their operation can vary in subtle ways:
Normally, a super cancel doesn’t require anything but the meter for the super itself.
But KOF in particular requires one extra stock of the meter for the cancel (except in the “counter mode” of ’99–2000, or the Power MAX of 2002; canceling from a command move does not carry this requirement. In XI, it takes one stock from the “skill gauge”).