The external form block compliments the built in form builder in concrete5 by giving you a flexible way to build forms with code. It comes with an example PHP file that shows the formatting a form must take. It should make complete sense to any PHP developer, but this is not a particularly useful block if you can't code. If you do need to reuse some encapsulated PHP throughout the site as a block, this is a great tool. If you're looking for an example, all the external forms are kept in the /blocks/external_form/forms/ directory of your website's file system.