The preamble is a block of text (actually, it's Phabricator "remarkup") which is displayed above the form when users are submitting or editing an object. What you are reading now is actually the preamble for the "Form configuration" form. The preamble you enter below will be displayed above the form you are currently editing, when users are using the form to submit a Phabricator object (task, paste, etc.)
The visibility policy for a given form controls who can see the form in their global "Create" menu. For default-edit forms the first visible form is the one that is used to submit new objects. The consequence of this ordering and visibility rules is useful in that you can set up multiple forms with different visibility policies in order to give different default forms to different groups of users.
So the simplest use-case for this is to show the "Submit X task" custom form to all members of project X, while everyone else gets a generic "Submit a task" form.