Originally just called Duckworth-Lewis (DL) , with the Stern part added in 2014, it’s a mathematical formulation designed to create a target score for the team batting second in matches where weather has affected the match to the extent that the team batting second simply won’t be able to bat the full allocation of overs. It was created to try to find a winner in games where rain meant there wasn’t enough time left in the match for teams to bat the original number of overs rather than having lots of games that ended without a result, which was particularly disruptive in tournaments like a World Cup.