Most users are already using AI 2 and are unaffected by this change. But if you are still using AI Classic (version 1), then this message is very important – please click through the link to read the full story!

May 24, 2015 — MIT App Inventor Classic will shut down on July 15, 2015. After July 15, 2015, you will not be able to access or edit your existing AI Classic projects. It will be impossible to create new AI Classic projects.This shutdown applies only to AI Classic (AI1). App Inventor 2 (AI2) will not be affected.

MIT will not automatically convert your AI1 projects to AI2 projects. But MIT will provide a conversion tool that will help you convert the AI projects that are important to you into AI2 projects. We expect to release this converter in about 2 weeks (i.e., around June 8).

Source: App Inventor Classic to Shut Down on July 15 | Explore MIT App Inventor

How do you know which version you are using?
Login to App Inventor and create or open a project. If the URL address in your browser begins with
http://ai2.appinventor.mit.edu/

then you are using Version 2.