Auto cancel is basically the aerial cancelling before it hits the ground. As far as I'm aware of, there's 2 types to it.
1. When an aerial hits the ground in its last frames, the move is considered finished and the character lands with no lag frames. I don't believe Ganondorf has these.
2. When an aerial is done so quickly that it finishes in the air and the character can land on the ground with no lag. Ganon can do this with bair, uair, and dair. Dair 'thunder-storming' is probably the most useful, though, as it deals the most damage and knockback.