LehrFEM++ is organized in modules that provide specific functionality that you can import in your code. In LehrFEM++ there is a strict 1:1 relation between modules and namespaces, folder structure, cmake targets, tests, include files and libraries which is best illustrated with an example. Consider the module with the name lf.mesh.hybrid2d

which consists of all C++ constructs in the namespace lf::mesh::hybrid2d,
whose source/header files all lie in the folder lib/lf/mesh/hybrid2d
which is built by the CMake target lf.mesh.hybrid2d
which provides the module include file lib/lf/mesh/hybrid2d/hybrid2d.h that includes all other (public) header files of the module.
which is compiled into the library liblf.mesh.hybrid2d.a (on linux)
which is tested by the test module lf.mesh.hybrid2d.test

Modules can depend on each other. In the example above, the lf.mesh.hybrid2d module depends on the lf.base, lf.mesh and lf.geometry module. This means in particular that:

The cmake target lf.mesh.hybrid2d depends on the lf.mesh cmake target (see the file lib/lf/mesh/hybrid2d/CMakeLists.txt). Since the lf.mesh depends on lf.base and lf.geometry it is not necessary to specify these dependecies also.
The header files in the folder lib/lf/mesh/hybrid2d include the module include files lib/lf/base/base.h and lib/lf/mesh/mesh.h (as necessary)

General remarks about modules:

The modules in LehrFEM++ are all libraries, the only exception being test modules and example modules (which are executables)
Don't try to include individual header files from other modules directly. Instead include the module header file (e.g. lib/lf/mesh/hybrid2d/hybrid2d.h) which will include all necessary header files for that module.
Within the same module you can of course include other header files directly, e.g. the file lib/lf/mesh/hybrid2d/entity.h can include lib/lf/mesh/hybrid2d/mesh.h but it should not include lib/lf/base/forward_range.h!

For a list of (library) modules in LehrFEM++ see the list of namespaces.