I this instance, money, they want Europe to pay for them keeping the seaways open.
The point is that America has enjoyed great influence over Europe for years because it was then, in their best interests (because they were paranoid about communism, Cuba, Vietnam?) but now that communism is gone, they perceive the threat to be more Asia based yet Russia, remains a threat to Europe so it's a pretty short sighted view.
America covered European security because it was in their interests to do so but even now, as they are on the verge of pulling out all support, they continue to strongarm Europe. The UK should tell them to leave the RAF base in Cyprus and see how that goes down.