This is the title of the main page of the "Content" menu item

What is this section about  

This section contains a main page and some subpages to illustrate the pages hierarchy, and also to have an overview of the objects an styles that can be used when editing typical websites content. Have a look in the sub pages for more information.

I want to get rid of all of this and start editing my website  

You're ready, you already know enough of siteNOTE and all the stuff here is not relevant for you anymore, before deleting this section you can start editing your site with your own content and your own look-and-feel, just keep this section for reference and style management while you're developing your site, you'll delete it at the end of the development process.

If ever this section should not appear in the menu for whatever reason, you can still hide it by using the "hidden page" property of the "Content" page. Accessing the page will remain possible by using the "View Hidden" checkbox of the top SiteNOTE toolbar. See User's guide for more information.

The Homepage, here called "SiteNOTE, the .NET CMS", cannot be deleted. It must be renamed, using the page properties button in the top SiteNOTE toolbar to something more relevant like "home" (let's be original), and its content has to be replaced by yours.