Reckoning usually means a calculation. In this context it can also mean revenge. The idea is that a person's bad deeds are calculated and paid for. It's a lot like calculating a bill at the end of a meal. In this case a person who has done something bad, has their bill calculated, and they pay the price. In a horror movie the idea might be that a group of people left someone to die, only they aren't dead. They are back to seek revenge--their reckoning. Mostly the word reckon is used to say "I think" something. You might say: I reckon we need to put some petrol in the car or we won't make it home.