An executive agreement is an agreement with another country that specifies how the Executive Department will handle certain laws/situations/etc. The key is that the executive agreement is limited to what is allowed under current laws. It's sort of along the same lines as Bush banning stem cell research with federal funds, then Obama allowing it. The federal law simply said "you have x dollars to spend on medical research." It is up to the President and the Executive Department to determine how that money will be spent. I don't know very much at all about this law, so I'm not sure what exactly they may be discussing, but it could be how tough the enforcement will be, specifics of international cooperation, etc.

The analogy I use is: In a calculus course, they don't spend a week teaching you how to take derivatives of position functions, then spend a week taking derivatives of velocity functions, then a week taking derivatives of cost functions. They teach you how to take derivatives, then expect you to be able to apply that knowledge in any situation that may require it.