Once the menu is chosen, typing the mnemonic is the same as clicking on the item. An accelerator is a series of keystrokes for accomplishing the function without using the menu at all. Functions include titles, exec, separator and menu. The program you want to start is lauched by exec. You can specify the full path name of the program or just its name, if your path variable is set correctly.
Menus are the sublevel menus.
Then, later in the listing, you have the definition for the Games Menu. Customizing your environment in Motif is easy and fun. Last but not least, you can use your Motif libraries to compile programs which are dynamically linked, which should make the binaries smaller and quicker. An example I suggest trying is NEdit. If you don't have Motif, you can still use the statically linked version of this editor, or try to compile it with Lesstif.