The command in step 1 generates a few header files and a source file. The header provides prototypes for all of the selected OpenGL functions, redefines them as function pointers, and defines all of the OpenGL constants as well. The source file provides initialization code for the function pointers as well as some other utility functions. We can include the glad/glad.h header file wherever we need prototypes for OpenGL functions, so all function entry points are available at compile time. At runtime, the gladLoadGL() call will initialize all available function pointers.
The command-line arguments available to GLAD are fully documented on the GitHub site and are available via glad -h. One can select any OpenGL version, select core/compatibility profiles, include desired extensions, and/or create debug callbacks.