Wikidot data forms make it possible to build simple applications into your wikidot sites. A normal wiki page holds unstructured text. A wiki page with a data form holds structured data ("fields"). Structured data is easier to edit, to understand, and to work with in many cases.
Wikidot stores pages in categories and when you use data forms, each page is one "object". So one category can have one data form, that applies to all pages in that category.
To create a new data form, edit the _template page in the category and add a [[form]] section.
See also the documentation on http://www.wikidot.com/doc:data-forms