MeatAxe modules are vector spaces acted on by MeatAxe matrix algebras. In the MeatAxe standalone these modules are described implicitly because the matrices contain all the necessary information there. In GAP the modules are the concrete objects whose properties are inspected (see Module Functions for MeatAxe Modules).
Note that most of the usual set theoreic and vector space functions are not Set Theoretic Functions for MeatAxe Modules, Vector Space Functions for MeatAxe Modules).
GAP 3.4.4