Briefly stated, module functors allow you to wrap up a set of functions and types as a module, and state not only which functions and types this module provides but also which functions and types the modules requires. You can plug together module functors with complementary provides and requires, like building with toy bricks.
Many languages support the provides half of this feature (e.g. Java's interfaces) but few support the requires half (Scheme's units are one of the few that come to mind). The advantage of supporting both provides and requires include ability to swap out equivalent implementations of some module, perform massive code abstraction, and to better isolate unit tests without resorting to conditional compilation. (You can quite literally plug a high-level module into a fake lower-level module in order to test the high-level one using one line of code.)
The "language of 2020" will have this, or an equivalent, means of massive abstraction. Or better still, such a mechanism will exist as a separate language unto itself which can work with multiple languages. (Think C-style symbolic linking combined with a .NET ABI.)