So, a meta-interface is one that is ``about the behavior and implementation of the primary interface.'' So, whereas the primary interface of a file system is about files, the meta-interface of a file system is about buffer sizes and caching policies.
This notion of ``meta'' is certainly not the only focus of work in the reflection community, but it is the primary one that we are borrowing. In particular, we take from Smith the concept of ``subject matter shift'' as a way of understanding what happens in discourse and what we need to capture in a well-designed base/meta separation [Smi82, Smi84].