An Out of Band Agreement is the set of rules and conventions that defined across an interface or API. These rules are not defined by the interface itself. The interface defines objects and methods to operate on data managed by an implementation of the interface. In many cases, there are restrictions on the data managed by the implementation that must be honored by the code that uses the implementation via the interface.
A few examples serve to illustrate this concept. The EduPerson object has a getAge() method that returns an integer. Ages have an implied range limit on them: 1 to 120 years. Negative ages are not meaningful. An age of zero is considered to be an undefined age. All of these restrictions are part of the Out of Band Agreement on the getAge() method.