I hope this doesn't take too long to explain but what is the reckoning? I never understood it

The Reckoning is what happens when the White King’s Scepter is activatesd (usually by the Black King) after the White King’s army has been defeated. It launches meteors from the Veil towards the Battlefield. Skaian Defense Portals redirect as many as they can but it is limited in how many it can redirect. The players have to defeat the Black King and stop the Reckoning before it destroys the Battlefield or they won’t be able to produce a Genesis Frog and win the game.