A main part is one for which you can request a generation step when the generation mode is “main part and its associates.” In the Workbench, you can right-click the file and select Generate, or you can edit the file and press Ctrl-G. The main part has the same name as the file in which the part resides.
A main part was formerly known as a “generatable” part.
You can request a generation step for the EGL deployment descriptor, too.
In most cases, the output for a main part is a stand-alone, compilable unit such as a program.