the best modules are those that provide powerful functionality yet have simple interfaces. the best modules are deep they allow a lot of functionality to be accessed through a simple interface. shallow modules one with a relatively complex interface, but not much functionality.
a good example of a deep module is the garbage collector in a language such as Java or Go. this module has no interface at all; it works invisibly behind the scenes to reclaim unused memory. adding garbage collection to a system actually shrinks its overall interface since it eliminates the interface for freeing objects. the implementation is quite complex but that complexity is hidden from programmers using the language.