When a character eats a combo, the game triggers a system called "damage scaling" which with after the first blow, every other guaranteed blow that hits the opponent in the same chain will starting to deal less and less damage until you let them stand to nullify the effect giving them a chance to defend.
When you intentionally drop the scaling in hope to start another combo, if you succeed the scale restarts from the beginning counting as a new combo, this is what a reset is.
The most recent character who could get a guaranteed combo from a reset was MK9 Cyrax due the unblockable bombs, he had ways to put you into it without chances of escaping making a freaking broke character with death combos on guaranteed inescapable resets.