In addition to the automated mesh improvement options, there are several manual diagnostic and repair tools that enable you to build mesh topology, remesh bad elements and fill holes easily. Also there are tools for the detection of overlapping triangles and non-manifold vertices, as well as detection of single/multiple edge and duplicate elements. Other tools allow you to merge or split the mesh, move elements, convert mesh type, change density, or otherwise manipulate the mesh.

These are fully described at Edit Mesh.