It means that when you use this effect to play a Digimon card, you must choose one with the DP reduced from the maximum shown by 2000 for each each card in your opponent's security stack. The fewer cards in your opponent's security stack, the greater the DP can be for the Digimon card played. For example, if your opponent has 3 security cards, the 13000 DP maximum is reduced by 6000, meaning that you can play a Digimon card with 7000 DP or less.