Normally you construct your GUI interactively with Glade and then choose to build the C code for the interface with the Build button. It is sometime inconvenient to fire up Glade just to build the C source code from the Glade XML file. Instead you can do the conversion from the command line with:
$ glade --write-source <gladefile>
If the command line option is not supplied, but a gladefile is supplied, then Glade will start up and load the specified project.