Flux lets you manage any sort of content you like, it doesn't prescribe that content = html + a title or anything like that so you have to define the structure and format of the content you want to manage on the site, and how the different types relate to each other.
eg: A news article might have a headline, some body content, a published date and an author whilst a simple html page might just have a title and some body content.
Once you've defined the types, you then have to decide how they are going to be edited and rendered to the browser by creating the templates for them.
There is some documentation on how to do this, http://www.fluxcms.co.uk/files/folders/32/download.aspx - it's not brilliant but it should get you started.