Or not pay.
In his story,
The big NATO country president: No, we won't pay, we're willing to be delinquent, are you going to protect us from Russia?
Trump: No. No pay no protection and I will encourage Russia do whatever they want.
Trump intended to let these free loaders know the consequences if they don't pay. This is how to force people who don't to pay in business.
Whatever you call the minimum 2% GDP defense spending for NATO members other than due (Trump call it "payment", others may call it "spending"), this spending indicates a country's political will to contribute to NATO's common defence efforts. More importantly, NATO defense is supposed to be alliance defense, i.e. a well coordinated well balanced defense. If one country, say Germany, doesn't spend enough to build, to upgrade and to develop their country's defense, Germany section will become volunable section, German's war readiness will be weak part of the whole NATO defense. Other NATO countries will have to make up this deficiency. Or enemy will take advantage of this weakness of NATO defense. Essentially this defense spending is a payment. You pay for your security.