Here is a list of all files with brief descriptions:

analys.c
analys_3d.c	Mesh analysis
anisomovpt.c	Functions to move a point in the mesh (with anisotropic metric)
anisomovpt_3d.c	Functions to move a point in the mesh
anisosiz.c	Fonctions for anisotropic size map computation
anisosiz_3d.c	Fonctions for anisotropic size map computation
API_functions.c	C API functions definitions for MMG library
API_functions_3d.c	C API functions definitions for MMG3D library
API_functionsf.c	Fortran API functions for MMG library
API_functionsf_3d.c	Fortran API functions for MMG3D library
bezier.c	Functions for Bezier surface computation
bezier_3d.c	Functions for Bezier surface computation
boulep.c	Functions for ball of points computation
boulep_3d.c	Functions for ball of points computation
cenrad_3d.c	Compute radius and center of circumscribing circle to the element
chkmsh_3d.c	Check the input mesh validity
chrono.c	Simulation of a chronograph
chrono.h
colver_3d.c	Functions for vertices collapsing
delaunay_3d.c	Functions for mesh modifications in Delaunay mode
eigenv.c	Find eigenvalues and eigenvectors of matrix
eigenv.h
hash.c	Functions for hash tables management and tetrahedra packing
hash_3d.c	Functions for hash tables management and tetrahedra packing
inlined_functions.h	Inlined Functions
inlined_functions_3d.h	Inlined Functions
inout.c	Input / Output Functions
inout_3d.c	Input / Output Functions
intmet.c	Functions to compute metric interpolation
intmet_3d.c	Metric interpolations
isosiz.c	Fonctions for isotropic size map computation
isosiz_3d.c	Fonctions for isotropic size map computation
libmmg3d.c	Most of the API functions of the MMG3D library
libmmg3d.h	API headers for the mmg3d library
libmmg3d_tools.c	Tools functions for the mmg3d library
libmmg3d_toolsf.c	Fortran API functions for MMG3D library
libmmg3df.c	Fortran API functions for MMG3D library
libmmgcommon.h	API header for the common part of the MMG libraries
libmmgtypes.h
librnbg.c	Functions for scotch renumerotation
librnbg.h	Header file for the librnbg.c librnbg_s.c librnbg_3d.c files
librnbg_3d.c	Functions for scotch renumerotation
mettools.c	Metric tools for the mmg applications
mmg.c	Common part for functions used in mmgs.c and mmg3d.c files
mmg2.c
mmg3d.c	Main file for MMG3D executable: perform 3d mesh adaptation
mmg3d.h
mmg3d1.c	Perform volume and surface mesh adaptation with pattern splitting
mmg3d1_delone.c	Perform volume and surface mesh adaptation in delaunay mode
mmg3d1_pattern.c	Perform volume and surface mesh adaptation with pattern splitting
mmg3d2.c	Create implicit surface in mesh
mmg3d3.c	Lagrangian meshing
mmg3dexterns.c
mmg3dexterns.h
mmgcommon.h
mmgexterns.c
mmgexterns.h
movpt_3d.c	Functions to move a point in the mesh
optbdry_3d.c	Functions for the optimization of very bad elements
optlap_3d.c	Functions for the optimization with laplacian/anti-laplacian
opttyp_3d.c	Functions for the optimization of very bad elements
PRoctree_3d.c
quality.c	Functions to compute elements quality and edge lengths
quality_3d.c	Functions to compute elements quality and edge lengths
scalem.c	Scale and unscale mesh and solution
split_3d.c	Functions to create new points
swap_3d.c	Functions for swapping process over boundary
swapgen_3d.c	Functions for swapping process inside the mesh
tools.c	Various tools for the mmg applications
tools_3d.c	Various algorithmic and algebraic tools
variadic_3d.c	C variadic functions definitions for MMG3D library
velextls_3d.c	Tools for interfacing mmg with LS, for extension of the displacement field
zaldy_3d.c	Memory management