MIT App Inventor is basically for creating Android Apps in your browser where you design how the app will look and function. Like fitting together puzzle pieces, you set how your app will behave to different events by simply signing in with your Gmail Account, so that the App Inventor server can store your work and help you keep track of projects.

It basically consists of two parts:

App Inventor Designer
App Inventor Block Editor

In App Inventor Designer you select the components for your app while in App Inventor Block Editor you assemble program blocks that specify how the components should behave visually, fitting pieces together like pieces of a puzzle. After completing the above phases you may run your app directly in your Android phone by connecting it to your computer or may run it on Android Emulator if you don't have an Android phone. Moreover you may even download your app (.apk file) and install and run it on your Android device directly.