Doc-Central is a tool to conveniently browse the documentation on your Debian system.
It looks like you are using a browser that does not handle frames properly. This is not a problem, but please note it makes browsing the documentation a bit more inconvenient.
If you click here, you will get a list of available sections. By selecting a section list of documents in that section will show up. You can also search the list of documents by entering a regular expression and clicking on the 'search' button. If you enter an invalid regular expression, a normal string search will be used instead.
You can view the documentation by selecting its title. If it is available in multiple formats the formats will be listed next to the title. Simply selecting the format of your choice will show the document in desired format.