It creates all of the necessary data types for your internal structures based on your input file.
Say you've got a bunch of different XML formats that you need to read into internal data structures. You could sit down and code a SAX parser from scratch. You could use a DOM parser and then wander through the DOM filling in your internal data structures. Or, you could write a little snippet describing how you'd like to turn your XML into structures and run this parser generator by Patrick Stein.
The #'MAIN function takes a list of command line arguments. See the src/generator/0README.org for information on the command-line arguments supported.