Resets, in normal fighting game terms, are meant to describe a situation where you (usually) cut a combo short, reducing your damage output and at the same time put your opponent in a situation where they must make a guess (high/low mixup, strike/throw mixup) allowing you to continue damage and/or pressure. If they guess wrong after being reset, you usually get another full combo.
In MK9 terms, resets are used to describe a situation where you end a combo, then start a new, (usually) inescapable combo with reset damage scaling and gravity...Cyrax and Smoke do this to obtain their high damage output making them so deadly.