The PCB Footprint Editor is a tool for editing the footprints, also known as "modules".
The footprint libraries used to be in .mod files. (Those files can still be read, but the footprints in them can't be edited. We're moving (1/17) to a new system where the footprint libraries consist of .kicad_mod files are held in XX.pretty folders.)
(Also- remember that .mod / .kicad_mod files are not the only libraries. A .lib file is the other sort of library; it is a library of schematic symbols.)
There is a button on KiCad's central project manager window to launch the PCB Footprint Editor.
(The old (somewhat or well before vers 4.0.4) path to the PCB Footprint Editor, then called the "module editor" was via a button on PCBnew's top toolbar.)
See also the Schematic Symbol Editor. It is to the schematic symbols in .lib files (the libraries of schematic symbols) what the PCB Footprint Editor is to footprints. (Footprints are also known as "modules".)