Opens and loads project files into FreeCAD's workspace and depending on the installed extension modules further file types are supported such as IGES, STEP, BREP, STL, OBJ, PLY and so on.
When loading files always a new document gets created. If you want to load a file into an existing document use the Std_Import instead.
FreeCAD project files have the extension "FCStd". To get more information about the internal layout of FreeCAD projects files read the article File Format FCStd.