These are one-off, special pages, coded in PHP, that also have a back-end "controller" component that maps to different actions that might happen on the front-end of a page. You'd create a form by hand in your single page template, and submit it to an action function, which is then automatically run in the controller. (This is not the best documentation of how these actually work.)
This is a pretty good introduction:
Now, this isn't an automated form building tool or anything like that, it's really just a syntax for integrating custom PHP code and stuff into your c5 applications..
Does this help at all?