You have to enter your Google AdSense ID on this site. Also, you now have access to a whole bunch of other coding stuff such as putting ads on websites you control. You can put adwords on sites you control... for instance, I have several domains I pay for and I put ads on them, as well as the ones placed here and on Youtube automagically.
And they provide widgets too. For instance, if you have a website where you can ad content in the form of html you can place a Google search box on the site and if someone uses it then it counts towards an extra penny or so.
It also gives you optimization codes to insert into your header on web sites. If you don't have other web sites (I suspect you don't or you wouldn't be asking this question) it means putting that little code in your website will help you show up better in Google searches.