You most probably got here by clicking on an email signature or web link that looks like: http://xri.net/=bartt.
http://xri.net/ is an XRI proxy; a service that bridges the lack of XRI-aware applications today. So, anytime you see an i-name, you can immediately use it by putting http://xri.net/ in front of it:
http://xri.net/ + =bartt → http://xri.net/=bartt
An i-name is an abstract identifier that identifies an entity. That entity could be a person or a company.
What happens when you resolve an i-name is that an “XRDS document” is retrieved. In a fully resolved XRDS document, the list of services available is most immediately useful. It describes services such as the OpenID endpoint, your contact page, blog, Skype ID... pretty much anything you want to tell the world about.
Here are the services I have in my XRDS. The first service is configured to be the default service if you did not specify what type of service you are looking for. It points to this page you're reading. You may also choose to make your contact page the default service.